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 <chapter id="protocol">
  <title>Frontend/Backend Protocol</title>

  <indexterm zone="protocol">
   <primary>protocol</primary>
   <secondary>frontend-backend</secondary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> uses a message-based protocol
   for communication between frontends and backends (clients and servers).
   The protocol is supported over <acronym>TCP/IP</acronym> and also over
   Unix-domain sockets.  Port number 5432 has been registered with IANA as
   the customary TCP port number for servers supporting this protocol, but
   in practice any non-privileged port number can be used.
  </para>

  <para>
   This document describes version 3.0 of the protocol, implemented in
   <productname>PostgreSQL</productname> 7.4 and later.  For descriptions
   of the earlier protocol versions, see previous releases of the
   <productname>PostgreSQL</productname> documentation.  A single server
   can support multiple protocol versions.  The initial startup-request
   message tells the server which protocol version the client is attempting to
   use.  If the major version requested by the client is not supported by
   the server, the connection will be rejected (for example, this would occur
   if the client requested protocol version 4.0, which does not exist as of
   this writing).  If the minor version requested by the client is not
   supported by the server (e.g., the client requests version 3.1, but the
   server supports only 3.0), the server may either reject the connection or
   may respond with a NegotiateProtocolVersion message containing the highest
   minor protocol version which it supports.  The client may then choose either
   to continue with the connection using the specified protocol version or
   to abort the connection.
  </para>

   <para>
    In order to serve multiple clients efficiently, the server launches
    a new <quote>backend</quote> process for each client.
    In the current implementation, a new child
    process is created immediately after an incoming connection is detected.
    This is transparent to the protocol, however.  For purposes of the
    protocol, the terms <quote>backend</quote> and <quote>server</quote> are
    interchangeable; likewise <quote>frontend</quote> and <quote>client</quote>
    are interchangeable.
   </para>

  <sect1 id="protocol-overview">
   <title>Overview</title>

   <para>
    The protocol has separate phases for startup and normal operation.
    In the startup phase, the frontend opens a connection to the server
    and authenticates itself to the satisfaction of the server.  (This might
    involve a single message, or multiple messages depending on the
    authentication method being used.)  If all goes well, the server then sends
    status information to the frontend, and finally enters normal operation.
    Except for the initial startup-request message, this part of the
    protocol is driven by the server.
   </para>

   <para>
    During normal operation, the frontend sends queries and
    other commands to the backend, and the backend sends back query results
    and other responses.  There are a few cases (such as <command>NOTIFY</command>)
    wherein the
    backend will send unsolicited messages, but for the most part this portion
    of a session is driven by frontend requests.
   </para>

   <para>
    Termination of the session is normally by frontend choice, but can be
    forced by the backend in certain cases.  In any case, when the backend
    closes the connection, it will roll back any open (incomplete) transaction
    before exiting.
   </para>

   <para>
    Within normal operation, SQL commands can be executed through either of
    two sub-protocols.  In the <quote>simple query</quote> protocol, the frontend
    just sends a textual query string, which is parsed and immediately
    executed by the backend.  In the <quote>extended query</quote> protocol,
    processing of queries is separated into multiple steps: parsing,
    binding of parameter values, and execution.  This offers flexibility
    and performance benefits, at the cost of extra complexity.
   </para>

   <para>
    Normal operation has additional sub-protocols for special operations
    such as <command>COPY</command>.
   </para>

  <sect2 id="protocol-message-concepts">
   <title>Messaging Overview</title>

   <para>
    All communication is through a stream of messages.  The first byte of a
    message identifies the message type, and the next four bytes specify the
    length of the rest of the message (this length count includes itself, but
    not the message-type byte).  The remaining contents of the message are
    determined by the message type.  For historical reasons, the very first
    message sent by the client (the startup message) has no initial
    message-type byte.
   </para>

   <para>
    To avoid losing synchronization with the message stream, both servers and
    clients typically read an entire message into a buffer (using the byte
    count) before attempting to process its contents.  This allows easy
    recovery if an error is detected while processing the contents.  In
    extreme situations (such as not having enough memory to buffer the
    message), the receiver can use the byte count to determine how much
    input to skip before it resumes reading messages.
   </para>

   <para>
    Conversely, both servers and clients must take care never to send an
    incomplete message.  This is commonly done by marshaling the entire message
    in a buffer before beginning to send it.  If a communications failure
    occurs partway through sending or receiving a message, the only sensible
    response is to abandon the connection, since there is little hope of
    recovering message-boundary synchronization.
   </para>
  </sect2>

   <sect2 id="protocol-query-concepts">
    <title>Extended Query Overview</title>

    <para>
     In the extended-query protocol, execution of SQL commands is divided
     into multiple steps.  The state retained between steps is represented
     by two types of objects: <firstterm>prepared statements</firstterm> and
     <firstterm>portals</firstterm>.  A prepared statement represents the result of
     parsing and semantic analysis of a textual query string.
     A prepared statement is not in itself ready to execute, because it might
     lack specific values for <firstterm>parameters</firstterm>.  A portal represents
     a ready-to-execute or already-partially-executed statement, with any
     missing parameter values filled in.  (For <command>SELECT</command> statements,
     a portal is equivalent to an open cursor, but we choose to use a different
     term since cursors don't handle non-<command>SELECT</command> statements.)
    </para>

    <para>
     The overall execution cycle consists of a <firstterm>parse</firstterm> step,
     which creates a prepared statement from a textual query string; a
     <firstterm>bind</firstterm> step, which creates a portal given a prepared
     statement and values for any needed parameters; and an
     <firstterm>execute</firstterm> step that runs a portal's query.  In the case of
     a query that returns rows (<command>SELECT</command>, <command>SHOW</command>, etc),
     the execute step can be told to fetch only
     a limited number of rows, so that multiple execute steps might be needed
     to complete the operation.
    </para>

    <para>
     The backend can keep track of multiple prepared statements and portals
     (but note that these exist only within a session, and are never shared
     across sessions).  Existing prepared statements and portals are
     referenced by names assigned when they were created.  In addition,
     an <quote>unnamed</quote> prepared statement and portal exist.  Although these
     behave largely the same as named objects, operations on them are optimized
     for the case of executing a query only once and then discarding it,
     whereas operations on named objects are optimized on the expectation
     of multiple uses.
    </para>
   </sect2>

   <sect2 id="protocol-format-codes">
    <title>Formats and Format Codes</title>

    <para>
     Data of a particular data type might be transmitted in any of several
     different <firstterm>formats</firstterm>.  As of <productname>PostgreSQL</productname> 7.4
     the only supported formats are <quote>text</quote> and <quote>binary</quote>,
     but the protocol makes provision for future extensions.  The desired
     format for any value is specified by a <firstterm>format code</firstterm>.
     Clients can specify a format code for each transmitted parameter value
     and for each column of a query result.  Text has format code zero,
     binary has format code one, and all other format codes are reserved
     for future definition.
    </para>

    <para>
     The text representation of values is whatever strings are produced
     and accepted by the input/output conversion functions for the
     particular data type.  In the transmitted representation, there is
     no trailing null character; the frontend must add one to received
     values if it wants to process them as C strings.
     (The text format does not allow embedded nulls, by the way.)
    </para>

    <para>
     Binary representations for integers use network byte order (most
     significant byte first).  For other data types consult the documentation
     or source code to learn about the binary representation.  Keep in mind
     that binary representations for complex data types might change across
     server versions; the text format is usually the more portable choice.
    </para>
   </sect2>
  </sect1>

  <sect1 id="protocol-flow">
   <title>Message Flow</title>

   <para>
    This section describes the message flow and the semantics of each
    message type.  (Details of the exact representation of each message
    appear in <xref linkend="protocol-message-formats"/>.)  There are
    several different sub-protocols depending on the state of the
    connection: start-up, query, function call,
    <command>COPY</command>, and termination.  There are also special
    provisions for asynchronous operations (including notification
    responses and command cancellation), which can occur at any time
    after the start-up phase.
   </para>

   <sect2>
    <title>Start-up</title>

    <para>
     To begin a session, a frontend opens a connection to the server and sends
     a startup message.  This message includes the names of the user and of the
     database the user wants to connect to; it also identifies the particular
     protocol version to be used.  (Optionally, the startup message can include
     additional settings for run-time parameters.)
     The server then uses this information and
     the contents of its configuration files (such as
     <filename>pg_hba.conf</filename>) to determine
     whether the connection is provisionally acceptable, and what additional
     authentication is required (if any).
    </para>

    <para>
     The server then sends an appropriate authentication request message,
     to which the frontend must reply with an appropriate authentication
     response message (such as a password).
     For all authentication methods except GSSAPI, SSPI and SASL, there is at
     most one request and one response. In some methods, no response
     at all is needed from the frontend, and so no authentication request
     occurs. For GSSAPI, SSPI and SASL, multiple exchanges of packets may be
     needed to complete the authentication.
    </para>

    <para>
     The authentication cycle ends with the server either rejecting the
     connection attempt (ErrorResponse), or sending AuthenticationOk.
    </para>

    <para>
     The possible messages from the server in this phase are:

     <variablelist>
      <varlistentry>
       <term>ErrorResponse</term>
       <listitem>
        <para>
         The connection attempt has been rejected.
         The server then immediately closes the connection.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationOk</term>
       <listitem>
        <para>
         The authentication exchange is successfully completed.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationKerberosV5</term>
       <listitem>
        <para>
         The frontend must now take part in a Kerberos V5
         authentication dialog (not described here, part of the
         Kerberos specification) with the server.  If this is
         successful, the server responds with an AuthenticationOk,
         otherwise it responds with an ErrorResponse. This is no
         longer supported.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationCleartextPassword</term>
       <listitem>
        <para>
         The frontend must now send a PasswordMessage containing the
         password in clear-text form.  If
         this is the correct password, the server responds with an
         AuthenticationOk, otherwise it responds with an ErrorResponse.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationMD5Password</term>
       <listitem>
        <para>
         The frontend must now send a PasswordMessage containing the
         password (with user name) encrypted via MD5, then encrypted
         again using the 4-byte random salt specified in the
         AuthenticationMD5Password message.  If this is the correct
         password, the server responds with an AuthenticationOk,
         otherwise it responds with an ErrorResponse.  The actual
         PasswordMessage can be computed in SQL as <literal>concat('md5',
         md5(concat(md5(concat(password, username)), random-salt)))</literal>.
         (Keep in mind the <function>md5()</function> function returns its
         result as a hex string.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationSCMCredential</term>
       <listitem>
        <para>
         This response is only possible for local Unix-domain connections
         on platforms that support SCM credential messages.  The frontend
         must issue an SCM credential message and then send a single data
         byte.  (The contents of the data byte are uninteresting; it's
         only used to ensure that the server waits long enough to receive
         the credential message.)  If the credential is acceptable,
         the server responds with an
         AuthenticationOk, otherwise it responds with an ErrorResponse.
         (This message type is only issued by pre-9.1 servers.  It may
         eventually be removed from the protocol specification.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationGSS</term>
       <listitem>
        <para>
         The frontend must now initiate a GSSAPI negotiation. The frontend
         will send a GSSResponse message with the first part of the GSSAPI
         data stream in response to this. If further messages are needed,
         the server will respond with AuthenticationGSSContinue.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationSSPI</term>
       <listitem>
        <para>
         The frontend must now initiate an SSPI negotiation. The frontend
         will send a GSSResponse with the first part of the SSPI
         data stream in response to this. If further messages are needed,
         the server will respond with AuthenticationGSSContinue.
        </para>
       </listitem>

      </varlistentry>
      <varlistentry>
       <term>AuthenticationGSSContinue</term>
       <listitem>
        <para>
         This message contains the response data from the previous step
         of GSSAPI or SSPI negotiation (AuthenticationGSS, AuthenticationSSPI
         or a previous AuthenticationGSSContinue). If the GSSAPI
         or SSPI data in this message
         indicates more data is needed to complete the authentication,
         the frontend must send that data as another GSSResponse message. If
         GSSAPI or SSPI authentication is completed by this message, the server
         will next send AuthenticationOk to indicate successful authentication
         or ErrorResponse to indicate failure.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationSASL</term>
       <listitem>
        <para>
         The frontend must now initiate a SASL negotiation, using one of the
         SASL mechanisms listed in the message. The frontend will send a
         SASLInitialResponse with the name of the selected mechanism, and the
         first part of the SASL data stream in response to this. If further
         messages are needed, the server will respond with
         AuthenticationSASLContinue. See <xref linkend="sasl-authentication"/>
         for details.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationSASLContinue</term>
       <listitem>
        <para>
         This message contains challenge data from the previous step of SASL
         negotiation (AuthenticationSASL, or a previous
         AuthenticationSASLContinue). The frontend must respond with a
         SASLResponse message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>AuthenticationSASLFinal</term>
       <listitem>
        <para>
         SASL authentication has completed with additional mechanism-specific
         data for the client. The server will next send AuthenticationOk to
         indicate successful authentication, or an ErrorResponse to indicate
         failure. This message is sent only if the SASL mechanism specifies
         additional data to be sent from server to client at completion.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>NegotiateProtocolVersion</term>
       <listitem>
        <para>
         The server does not support the minor protocol version requested
         by the client, but does support an earlier version of the protocol;
         this message indicates the highest supported minor version.  This
         message will also be sent if the client requested unsupported protocol
         options (i.e., beginning with <literal>_pq_.</literal>) in the
         startup packet.  This message will be followed by an ErrorResponse or
         a message indicating the success or failure of authentication.
        </para>
       </listitem>
      </varlistentry>

     </variablelist>
    </para>

    <para>
     If the frontend does not support the authentication method
     requested by the server, then it should immediately close the
     connection.
    </para>

    <para>
     After having received AuthenticationOk, the frontend must wait
     for further messages from the server.  In this phase a backend process
     is being started, and the frontend is just an interested bystander.
     It is still possible for the startup attempt
     to fail (ErrorResponse) or the server to decline support for the requested
     minor protocol version (NegotiateProtocolVersion), but in the normal case
     the backend will send some ParameterStatus messages, BackendKeyData, and
     finally ReadyForQuery.
    </para>

    <para>
     During this phase the backend will attempt to apply any additional
     run-time parameter settings that were given in the startup message.
     If successful, these values become session defaults.  An error causes
     ErrorResponse and exit.
    </para>

    <para>
     The possible messages from the backend in this phase are:

     <variablelist>
      <varlistentry>
       <term>BackendKeyData</term>
       <listitem>
        <para>
         This message provides secret-key data that the frontend must
         save if it wants to be able to issue cancel requests later.
         The frontend should not respond to this message, but should
         continue listening for a ReadyForQuery message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>ParameterStatus</term>
       <listitem>
        <para>
         This message informs the frontend about the current (initial)
          setting of backend parameters, such as <xref
          linkend="guc-client-encoding"/> or <xref linkend="guc-datestyle"/>.
          The frontend can ignore this message, or record the settings
          for its future use; see <xref linkend="protocol-async"/> for
          more details.  The frontend should not respond to this
          message, but should continue listening for a ReadyForQuery
          message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>ReadyForQuery</term>
       <listitem>
        <para>
         Start-up is completed.  The frontend can now issue commands.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>ErrorResponse</term>
       <listitem>
        <para>
         Start-up failed.  The connection is closed after sending this
         message.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>NoticeResponse</term>
       <listitem>
        <para>
         A warning message has been issued.  The frontend should
         display the message but continue listening for ReadyForQuery
         or ErrorResponse.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

    <para>
     The ReadyForQuery message is the same one that the backend will
     issue after each command cycle.  Depending on the coding needs of
     the frontend, it is reasonable to consider ReadyForQuery as
     starting a command cycle, or to consider ReadyForQuery as ending the
     start-up phase and each subsequent command cycle.
    </para>
   </sect2>

   <sect2>
    <title>Simple Query</title>

    <para>
     A simple query cycle is initiated by the frontend sending a Query message
     to the backend.  The message includes an SQL command (or commands)
     expressed as a text string.
     The backend then sends one or more response
     messages depending on the contents of the query command string,
     and finally a ReadyForQuery response message.  ReadyForQuery
     informs the frontend that it can safely send a new command.
     (It is not actually necessary for the frontend to wait for
     ReadyForQuery before issuing another command, but the frontend must
     then take responsibility for figuring out what happens if the earlier
     command fails and already-issued later commands succeed.)
    </para>

    <para>
     The possible response messages from the backend are:

     <variablelist>
      <varlistentry>
       <term>CommandComplete</term>
       <listitem>
        <para>
         An SQL command completed normally.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>CopyInResponse</term>
       <listitem>
        <para>
         The backend is ready to copy data from the frontend to a
         table; see <xref linkend="protocol-copy"/>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>CopyOutResponse</term>
       <listitem>
        <para>
         The backend is ready to copy data from a table to the
         frontend; see <xref linkend="protocol-copy"/>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>RowDescription</term>
       <listitem>
        <para>
         Indicates that rows are about to be returned in response to
         a <command>SELECT</command>, <command>FETCH</command>, etc query.
         The contents of this message describe the column layout of the rows.
         This will be followed by a DataRow message for each row being returned
         to the frontend.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>DataRow</term>
       <listitem>
        <para>
         One of the set of rows returned by
         a <command>SELECT</command>, <command>FETCH</command>, etc query.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>EmptyQueryResponse</term>
       <listitem>
        <para>
         An empty query string was recognized.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>ErrorResponse</term>
       <listitem>
        <para>
         An error has occurred.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>ReadyForQuery</term>
       <listitem>
        <para>
         Processing of the query string is complete.  A separate
         message is sent to indicate this because the query string might
         contain multiple SQL commands.  (CommandComplete marks the
         end of processing one SQL command, not the whole string.)
         ReadyForQuery will always be sent, whether processing
         terminates successfully or with an error.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>NoticeResponse</term>
       <listitem>
        <para>
         A warning message has been issued in relation to the query.
         Notices are in addition to other responses, i.e., the backend
         will continue processing the command.
        </para>
       </listitem>
      </varlistentry>

     </variablelist>
    </para>

    <para>
     The response to a <command>SELECT</command> query (or other queries that
     return row sets, such as <command>EXPLAIN</command> or <command>SHOW</command>)
     normally consists of RowDescription, zero or more
     DataRow messages, and then CommandComplete.
     <command>COPY</command> to or from the frontend invokes special protocol
     as described in <xref linkend="protocol-copy"/>.
     All other query types normally produce only
     a CommandComplete message.
    </para>

    <para>
     Since a query string could contain several queries (separated by
     semicolons), there might be several such response sequences before the
     backend finishes processing the query string.  ReadyForQuery is issued
     when the entire string has been processed and the backend is ready to
     accept a new query string.
    </para>

    <para>
     If a completely empty (no contents other than whitespace) query string
     is received, the response is EmptyQueryResponse followed by ReadyForQuery.
    </para>

    <para>
     In the event of an error, ErrorResponse is issued followed by
     ReadyForQuery.  All further processing of the query string is aborted by
     ErrorResponse (even if more queries remained in it).  Note that this
     might occur partway through the sequence of messages generated by an
     individual query.
    </para>

    <para>
     In simple Query mode, the format of retrieved values is always text,
     except when the given command is a <command>FETCH</command> from a cursor
     declared with the <literal>BINARY</literal> option.  In that case, the
     retrieved values are in binary format.  The format codes given in
     the RowDescription message tell which format is being used.
    </para>

    <para>
     A frontend must be prepared to accept ErrorResponse and
     NoticeResponse messages whenever it is expecting any other type of
     message.  See also <xref linkend="protocol-async"/> concerning messages
     that the backend might generate due to outside events.
    </para>

    <para>
     Recommended practice is to code frontends in a state-machine style
     that will accept any message type at any time that it could make sense,
     rather than wiring in assumptions about the exact sequence of messages.
    </para>

    <sect3 id="protocol-flow-multi-statement">
     <title>Multiple Statements in a Simple Query</title>

     <para>
      When a simple Query message contains more than one SQL statement
      (separated by semicolons), those statements are executed as a single
      transaction, unless explicit transaction control commands are included
      to force a different behavior.  For example, if the message contains
 <programlisting>
 INSERT INTO mytable VALUES(1);
 SELECT 1/0;
 INSERT INTO mytable VALUES(2);
 </programlisting>
      then the divide-by-zero failure in the <command>SELECT</command> will force
      rollback of the first <command>INSERT</command>.  Furthermore, because
      execution of the message is abandoned at the first error, the second
      <command>INSERT</command> is never attempted at all.
     </para>

     <para>
      If instead the message contains
 <programlisting>
 BEGIN;
 INSERT INTO mytable VALUES(1);
 COMMIT;
 INSERT INTO mytable VALUES(2);
 SELECT 1/0;
 </programlisting>
      then the first <command>INSERT</command> is committed by the
      explicit <command>COMMIT</command> command.  The second <command>INSERT</command>
      and the <command>SELECT</command> are still treated as a single transaction,
      so that the divide-by-zero failure will roll back the
      second <command>INSERT</command>, but not the first one.
     </para>

     <para>
      This behavior is implemented by running the statements in a
      multi-statement Query message in an <firstterm>implicit transaction
      block</firstterm> unless there is some explicit transaction block for them to
      run in.  The main difference between an implicit transaction block and
      a regular one is that an implicit block is closed automatically at the
      end of the Query message, either by an implicit commit if there was no
      error, or an implicit rollback if there was an error.  This is similar
      to the implicit commit or rollback that happens for a statement
      executed by itself (when not in a transaction block).
     </para>

     <para>
      If the session is already in a transaction block, as a result of
      a <command>BEGIN</command> in some previous message, then the Query message
      simply continues that transaction block, whether the message contains
      one statement or several.  However, if the Query message contains
      a <command>COMMIT</command> or <command>ROLLBACK</command> closing the existing
      transaction block, then any following statements are executed in an
      implicit transaction block.
      Conversely, if a <command>BEGIN</command> appears in a multi-statement Query
      message, then it starts a regular transaction block that will only be
      terminated by an explicit <command>COMMIT</command> or <command>ROLLBACK</command>,
      whether that appears in this Query message or a later one.
      If the <command>BEGIN</command> follows some statements that were executed as
      an implicit transaction block, those statements are not immediately
      committed; in effect, they are retroactively included into the new
      regular transaction block.
     </para>

     <para>
      A <command>COMMIT</command> or <command>ROLLBACK</command> appearing in an implicit
      transaction block is executed as normal, closing the implicit block;
      however, a warning will be issued since a <command>COMMIT</command>
      or <command>ROLLBACK</command> without a previous <command>BEGIN</command> might
      represent a mistake.  If more statements follow, a new implicit
      transaction block will be started for them.
     </para>

     <para>
      Savepoints are not allowed in an implicit transaction block, since
      they would conflict with the behavior of automatically closing the
      block upon any error.
     </para>

     <para>
      Remember that, regardless of any transaction control commands that may
      be present, execution of the Query message stops at the first error.
      Thus for example given
 <programlisting>
 BEGIN;
 SELECT 1/0;
 ROLLBACK;
 </programlisting>
      in a single Query message, the session will be left inside a failed
      regular transaction block, since the <command>ROLLBACK</command> is not
      reached after the divide-by-zero error.  Another <command>ROLLBACK</command>
      will be needed to restore the session to a usable state.
     </para>

     <para>
      Another behavior of note is that initial lexical and syntactic
      analysis is done on the entire query string before any of it is
      executed.  Thus simple errors (such as a misspelled keyword) in later
      statements can prevent execution of any of the statements.  This
      is normally invisible to users since the statements would all roll
      back anyway when done as an implicit transaction block.  However,
      it can be visible when attempting to do multiple transactions within a
      multi-statement Query.  For instance, if a typo turned our previous
      example into
 <programlisting>
 BEGIN;
 INSERT INTO mytable VALUES(1);
 COMMIT;
 INSERT INTO mytable VALUES(2);
 SELCT 1/0;<!-- this typo is intentional -->
 </programlisting>
      then none of the statements would get run, resulting in the visible
      difference that the first <command>INSERT</command> is not committed.
      Errors detected at semantic analysis or later, such as a misspelled
      table or column name, do not have this effect.
     </para>
    </sect3>
   </sect2>

   <sect2 id="protocol-flow-ext-query">
    <title>Extended Query</title>

    <para>
     The extended query protocol breaks down the above-described simple
     query protocol into multiple steps.  The results of preparatory
     steps can be re-used multiple times for improved efficiency.
     Furthermore, additional features are available, such as the possibility
     of supplying data values as separate parameters instead of having to
     insert them directly into a query string.
    </para>

    <para>
     In the extended protocol, the frontend first sends a Parse message,
     which contains a textual query string, optionally some information
     about data types of parameter placeholders, and the
     name of a destination prepared-statement object (an empty string
     selects the unnamed prepared statement).  The response is
     either ParseComplete or ErrorResponse.  Parameter data types can be
     specified by OID; if not given, the parser attempts to infer the
     data types in the same way as it would do for untyped literal string
     constants.
    </para>

    <note>
     <para>
      A parameter data type can be left unspecified by setting it to zero,
      or by making the array of parameter type OIDs shorter than the
      number of parameter symbols (<literal>$</literal><replaceable>n</replaceable>)
      used in the query string.  Another special case is that a parameter's
      type can be specified as <type>void</type> (that is, the OID of the
      <type>void</type> pseudo-type).  This is meant to allow parameter symbols
      to be used for function parameters that are actually OUT parameters.
      Ordinarily there is no context in which a <type>void</type> parameter
      could be used, but if such a parameter symbol appears in a function's
      parameter list, it is effectively ignored.  For example, a function
      call such as <literal>foo($1,$2,$3,$4)</literal> could match a function with
      two IN and two OUT arguments, if <literal>$3</literal> and <literal>$4</literal>
      are specified as having type <type>void</type>.
     </para>
    </note>

    <note>
     <para>
      The query string contained in a Parse message cannot include more
      than one SQL statement; else a syntax error is reported.  This
      restriction does not exist in the simple-query protocol, but it
      does exist in the extended protocol, because allowing prepared
      statements or portals to contain multiple commands would complicate
      the protocol unduly.
     </para>
    </note>

    <para>
     If successfully created, a named prepared-statement object lasts till
     the end of the current session, unless explicitly destroyed.  An unnamed
     prepared statement lasts only until the next Parse statement specifying
     the unnamed statement as destination is issued.  (Note that a simple
     Query message also destroys the unnamed statement.)  Named prepared
     statements must be explicitly closed before they can be redefined by
     another Parse message, but this is not required for the unnamed statement.
     Named prepared statements can also be created and accessed at the SQL
     command level, using <command>PREPARE</command> and <command>EXECUTE</command>.
    </para>

    <para>
     Once a prepared statement exists, it can be readied for execution using a
     Bind message.  The Bind message gives the name of the source prepared
     statement (empty string denotes the unnamed prepared statement), the name
     of the destination portal (empty string denotes the unnamed portal), and
     the values to use for any parameter placeholders present in the prepared
     statement.  The
     supplied parameter set must match those needed by the prepared statement.
     (If you declared any <type>void</type> parameters in the Parse message,
     pass NULL values for them in the Bind message.)
     Bind also specifies the format to use for any data returned
     by the query; the format can be specified overall, or per-column.
     The response is either BindComplete or ErrorResponse.
    </para>

    <note>
     <para>
      The choice between text and binary output is determined by the format
      codes given in Bind, regardless of the SQL command involved.  The
      <literal>BINARY</literal> attribute in cursor declarations is irrelevant when
      using extended query protocol.
     </para>
    </note>

    <para>
     Query planning typically occurs when the Bind message is processed.
     If the prepared statement has no parameters, or is executed repeatedly,
     the server might save the created plan and re-use it during subsequent
     Bind messages for the same prepared statement.  However, it will do so
     only if it finds that a generic plan can be created that is not much
     less efficient than a plan that depends on the specific parameter values
     supplied.  This happens transparently so far as the protocol is concerned.
    </para>

    <para>
     If successfully created, a named portal object lasts till the end of the
     current transaction, unless explicitly destroyed.  An unnamed portal is
     destroyed at the end of the transaction, or as soon as the next Bind
     statement specifying the unnamed portal as destination is issued.  (Note
     that a simple Query message also destroys the unnamed portal.)  Named
     portals must be explicitly closed before they can be redefined by another
     Bind message, but this is not required for the unnamed portal.
     Named portals can also be created and accessed at the SQL
     command level, using <command>DECLARE CURSOR</command> and <command>FETCH</command>.
    </para>

    <para>
     Once a portal exists, it can be executed using an Execute message.
     The Execute message specifies the portal name (empty string denotes the
     unnamed portal) and
     a maximum result-row count (zero meaning <quote>fetch all rows</quote>).
     The result-row count is only meaningful for portals
     containing commands that return row sets; in other cases the command is
     always executed to completion, and the row count is ignored.
     The possible
     responses to Execute are the same as those described above for queries
     issued via simple query protocol, except that Execute doesn't cause
     ReadyForQuery or RowDescription to be issued.
    </para>

    <para>
     If Execute terminates before completing the execution of a portal
     (due to reaching a nonzero result-row count), it will send a
     PortalSuspended message; the appearance of this message tells the frontend
     that another Execute should be issued against the same portal to
     complete the operation.  The CommandComplete message indicating
     completion of the source SQL command is not sent until
     the portal's execution is completed.  Therefore, an Execute phase is
     always terminated by the appearance of exactly one of these messages:
     CommandComplete, EmptyQueryResponse (if the portal was created from
     an empty query string), ErrorResponse, or PortalSuspended.
    </para>

    <para>
     At completion of each series of extended-query messages, the frontend
     should issue a Sync message.  This parameterless message causes the
     backend to close the current transaction if it's not inside a
     <command>BEGIN</command>/<command>COMMIT</command> transaction block (<quote>close</quote>
     meaning to commit if no error, or roll back if error).  Then a
     ReadyForQuery response is issued.  The purpose of Sync is to provide
     a resynchronization point for error recovery.  When an error is detected
     while processing any extended-query message, the backend issues
     ErrorResponse, then reads and discards messages until a Sync is reached,
     then issues ReadyForQuery and returns to normal message processing.
     (But note that no skipping occurs if an error is detected
     <emphasis>while</emphasis> processing Sync &mdash; this ensures that there is one
     and only one ReadyForQuery sent for each Sync.)
    </para>

    <note>
     <para>
      Sync does not cause a transaction block opened with <command>BEGIN</command>
      to be closed.  It is possible to detect this situation since the
      ReadyForQuery message includes transaction status information.
     </para>
    </note>

    <para>
     In addition to these fundamental, required operations, there are several
     optional operations that can be used with extended-query protocol.
    </para>

    <para>
     The Describe message (portal variant) specifies the name of an existing
     portal (or an empty string for the unnamed portal).  The response is a
     RowDescription message describing the rows that will be returned by
     executing the portal; or a NoData message if the portal does not contain a
     query that will return rows; or ErrorResponse if there is no such portal.
    </para>

    <para>
     The Describe message (statement variant) specifies the name of an existing
     prepared statement (or an empty string for the unnamed prepared
     statement).  The response is a ParameterDescription message describing the
     parameters needed by the statement, followed by a RowDescription message
     describing the rows that will be returned when the statement is eventually
     executed (or a NoData message if the statement will not return rows).
     ErrorResponse is issued if there is no such prepared statement.  Note that
     since Bind has not yet been issued, the formats to be used for returned
     columns are not yet known to the backend; the format code fields in the
     RowDescription message will be zeroes in this case.
    </para>

    <tip>
     <para>
      In most scenarios the frontend should issue one or the other variant
      of Describe before issuing Execute, to ensure that it knows how to
      interpret the results it will get back.
     </para>
    </tip>

    <para>
     The Close message closes an existing prepared statement or portal
     and releases resources.  It is not an error to issue Close against
     a nonexistent statement or portal name.  The response is normally
     CloseComplete, but could be ErrorResponse if some difficulty is
     encountered while releasing resources.  Note that closing a prepared
     statement implicitly closes any open portals that were constructed
     from that statement.
    </para>

    <para>
     The Flush message does not cause any specific output to be generated,
     but forces the backend to deliver any data pending in its output
     buffers.  A Flush must be sent after any extended-query command except
     Sync, if the frontend wishes to examine the results of that command before
     issuing more commands.  Without Flush, messages returned by the backend
     will be combined into the minimum possible number of packets to minimize
     network overhead.
    </para>

    <note>
     <para>
      The simple Query message is approximately equivalent to the series Parse,
      Bind, portal Describe, Execute, Close, Sync, using the unnamed prepared
      statement and portal objects and no parameters.  One difference is that
      it will accept multiple SQL statements in the query string, automatically
      performing the bind/describe/execute sequence for each one in succession.
      Another difference is that it will not return ParseComplete, BindComplete,
      CloseComplete, or NoData messages.
     </para>
    </note>
   </sect2>

   <sect2 id="protocol-flow-pipelining">
    <title>Pipelining</title>

    <indexterm zone="protocol-flow-pipelining">
     <primary>pipelining</primary>
     <secondary>protocol specification</secondary>
    </indexterm>

    <para>
     Use of the extended query protocol
     allows <firstterm>pipelining</firstterm>, which means sending a series
     of queries without waiting for earlier ones to complete.  This reduces
     the number of network round trips needed to complete a given series of
     operations.  However, the user must carefully consider the required
     behavior if one of the steps fails, since later queries will already
     be in flight to the server.
    </para>

    <para>
     One way to deal with that is to make the whole query series be a
     single transaction, that is wrap it in <command>BEGIN</command> ...
     <command>COMMIT</command>.  However, this does not help if one wishes
     for some of the commands to commit independently of others.
    </para>

    <para>
     The extended query protocol provides another way to manage this
     concern, which is to omit sending Sync messages between steps that
     are dependent.  Since, after an error, the backend will skip command
     messages until it finds Sync, this allows later commands in a pipeline
     to be skipped automatically when an earlier one fails, without the
     client having to manage that explicitly with <command>BEGIN</command>
     and <command>COMMIT</command>.  Independently-committable segments
     of the pipeline can be separated by Sync messages.
    </para>

    <para>
     If the client has not issued an explicit <command>BEGIN</command>,
     then each Sync ordinarily causes an implicit <command>COMMIT</command>
     if the preceding step(s) succeeded, or an
     implicit <command>ROLLBACK</command> if they failed.  However, there
     are a few DDL commands (such as <command>CREATE DATABASE</command>)
     that cannot be executed inside a transaction block.  If one of
     these is executed in a pipeline, it will, upon success, force an
     immediate commit to preserve database consistency.
     A Sync immediately following one of these has no effect except to
     respond with ReadyForQuery.
    </para>

    <para>
     When using this method, completion of the pipeline must be determined
     by counting ReadyForQuery messages and waiting for that to reach the
     number of Syncs sent.  Counting command completion responses is
     unreliable, since some of the commands may not be executed and thus not
     produce a completion message.
    </para>
   </sect2>

   <sect2>
    <title>Function Call</title>

    <para>
     The Function Call sub-protocol allows the client to request a direct
     call of any function that exists in the database's
     <structname>pg_proc</structname> system catalog.  The client must have
     execute permission for the function.
    </para>

    <note>
     <para>
      The Function Call sub-protocol is a legacy feature that is probably best
      avoided in new code.  Similar results can be accomplished by setting up
      a prepared statement that does <literal>SELECT function($1, ...)</literal>.
      The Function Call cycle can then be replaced with Bind/Execute.
     </para>
    </note>

    <para>
     A Function Call cycle is initiated by the frontend sending a
     FunctionCall message to the backend.  The backend then sends one
     or more response messages depending on the results of the function
     call, and finally a ReadyForQuery response message.  ReadyForQuery
     informs the frontend that it can safely send a new query or
     function call.
    </para>

    <para>
     The possible response messages from the backend are:

     <variablelist>
      <varlistentry>
       <term>ErrorResponse</term>
       <listitem>
        <para>
         An error has occurred.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>FunctionCallResponse</term>
       <listitem>
        <para>
         The function call was completed and returned the result given
         in the message.
         (Note that the Function Call protocol can only handle a single
         scalar result, not a row type or set of results.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>ReadyForQuery</term>
       <listitem>
        <para>
         Processing of the function call is complete.  ReadyForQuery
         will always be sent, whether processing terminates
         successfully or with an error.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>NoticeResponse</term>
       <listitem>
        <para>
         A warning message has been issued in relation to the function
         call.  Notices are in addition to other responses, i.e., the
         backend will continue processing the command.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>
   </sect2>

   <sect2 id="protocol-copy">
    <title>COPY Operations</title>

    <para>
     The <command>COPY</command> command allows high-speed bulk data transfer
     to or from the server.  Copy-in and copy-out operations each switch
     the connection into a distinct sub-protocol, which lasts until the
     operation is completed.
    </para>

    <para>
     Copy-in mode (data transfer to the server) is initiated when the
     backend executes a <command>COPY FROM STDIN</command> SQL statement.  The backend
     sends a CopyInResponse message to the frontend.  The frontend should
     then send zero or more CopyData messages, forming a stream of input
     data.  (The message boundaries are not required to have anything to do
     with row boundaries, although that is often a reasonable choice.)
     The frontend can terminate the copy-in mode by sending either a CopyDone
     message (allowing successful termination) or a CopyFail message (which
     will cause the <command>COPY</command> SQL statement to fail with an
     error).  The backend then reverts to the command-processing mode it was
     in before the <command>COPY</command> started, which will be either simple or
     extended query protocol.  It will next send either CommandComplete
     (if successful) or ErrorResponse (if not).
    </para>

    <para>
     In the event of a backend-detected error during copy-in mode (including
     receipt of a CopyFail message), the backend will issue an ErrorResponse
     message.  If the <command>COPY</command> command was issued via an extended-query
     message, the backend will now discard frontend messages until a Sync
     message is received, then it will issue ReadyForQuery and return to normal
     processing.  If the <command>COPY</command> command was issued in a simple
     Query message, the rest of that message is discarded and ReadyForQuery
     is issued.  In either case, any subsequent CopyData, CopyDone, or CopyFail
     messages issued by the frontend will simply be dropped.
    </para>

    <para>
     The backend will ignore Flush and Sync messages received during copy-in
     mode.  Receipt of any other non-copy message type constitutes an error
     that will abort the copy-in state as described above.  (The exception for
     Flush and Sync is for the convenience of client libraries that always
     send Flush or Sync after an Execute message, without checking whether
     the command to be executed is a <command>COPY FROM STDIN</command>.)
    </para>

    <para>
     Copy-out mode (data transfer from the server) is initiated when the
     backend executes a <command>COPY TO STDOUT</command> SQL statement.  The backend
     sends a CopyOutResponse message to the frontend, followed by
     zero or more CopyData messages (always one per row), followed by CopyDone.
     The backend then reverts to the command-processing mode it was
     in before the <command>COPY</command> started, and sends CommandComplete.
     The frontend cannot abort the transfer (except by closing the connection
     or issuing a Cancel request),
     but it can discard unwanted CopyData and CopyDone messages.
    </para>

    <para>
     In the event of a backend-detected error during copy-out mode,
     the backend will issue an ErrorResponse message and revert to normal
     processing.  The frontend should treat receipt of ErrorResponse as
     terminating the copy-out mode.
    </para>

    <para>
     It is possible for NoticeResponse and ParameterStatus messages to be
     interspersed between CopyData messages; frontends must handle these cases,
     and should be prepared for other asynchronous message types as well (see
     <xref linkend="protocol-async"/>).  Otherwise, any message type other than
     CopyData or CopyDone may be treated as terminating copy-out mode.
    </para>

    <para>
     There is another Copy-related mode called copy-both, which allows
     high-speed bulk data transfer to <emphasis>and</emphasis> from the server.
     Copy-both mode is initiated when a backend in walsender mode
     executes a <command>START_REPLICATION</command> statement.  The
     backend sends a CopyBothResponse message to the frontend.  Both
     the backend and the frontend may then send CopyData messages
     until either end sends a CopyDone message. After the client
     sends a CopyDone message, the connection goes from copy-both mode to
     copy-out mode, and the client may not send any more CopyData messages.
     Similarly, when the server sends a CopyDone message, the connection
     goes into copy-in mode, and the server may not send any more CopyData
     messages. After both sides have sent a CopyDone message, the copy mode
     is terminated, and the backend reverts to the command-processing mode.
     In the event of a backend-detected error during copy-both mode,
     the backend will issue an ErrorResponse message, discard frontend messages
     until a Sync message is received, and then issue ReadyForQuery and return
     to normal processing.  The frontend should treat receipt of ErrorResponse
     as terminating the copy in both directions; no CopyDone should be sent
     in this case.  See <xref linkend="protocol-replication"/> for more
     information on the subprotocol transmitted over copy-both mode.
    </para>

    <para>
     The CopyInResponse, CopyOutResponse and CopyBothResponse messages
     include fields that inform the frontend of the number of columns
     per row and the format codes being used for each column.  (As of
     the present implementation, all columns in a given <command>COPY</command>
     operation will use the same format, but the message design does not
     assume this.)
    </para>

   </sect2>

   <sect2 id="protocol-async">
    <title>Asynchronous Operations</title>

    <para>
     There are several cases in which the backend will send messages that
     are not specifically prompted by the frontend's command stream.
     Frontends must be prepared to deal with these messages at any time,
     even when not engaged in a query.
     At minimum, one should check for these cases before beginning to
     read a query response.
    </para>

    <para>
     It is possible for NoticeResponse messages to be generated due to
     outside activity; for example, if the database administrator commands
     a <quote>fast</quote> database shutdown, the backend will send a NoticeResponse
     indicating this fact before closing the connection.  Accordingly,
     frontends should always be prepared to accept and display NoticeResponse
     messages, even when the connection is nominally idle.
    </para>

    <para>
     ParameterStatus messages will be generated whenever the active
     value changes for any of the parameters the backend believes the
     frontend should know about.  Most commonly this occurs in response
     to a <command>SET</command> SQL command executed by the frontend, and
     this case is effectively synchronous &mdash; but it is also possible
     for parameter status changes to occur because the administrator
     changed a configuration file and then sent the
     <systemitem>SIGHUP</systemitem> signal to the server.  Also,
     if a <command>SET</command> command is rolled back, an appropriate
     ParameterStatus message will be generated to report the current
     effective value.
    </para>

    <para>
     At present there is a hard-wired set of parameters for which
     ParameterStatus will be generated: they are
     <varname>server_version</varname>,
     <varname>server_encoding</varname>,
     <varname>client_encoding</varname>,
     <varname>application_name</varname>,
     <varname>default_transaction_read_only</varname>,
     <varname>in_hot_standby</varname>,
     <varname>is_superuser</varname>,
     <varname>session_authorization</varname>,
     <varname>DateStyle</varname>,
     <varname>IntervalStyle</varname>,
     <varname>TimeZone</varname>,
     <varname>integer_datetimes</varname>, and
     <varname>standard_conforming_strings</varname>.
     (<varname>server_encoding</varname>, <varname>TimeZone</varname>, and
     <varname>integer_datetimes</varname> were not reported by releases before 8.0;
     <varname>standard_conforming_strings</varname> was not reported by releases
     before 8.1;
     <varname>IntervalStyle</varname> was not reported by releases before 8.4;
     <varname>application_name</varname> was not reported by releases before
     9.0;
     <varname>default_transaction_read_only</varname> and
     <varname>in_hot_standby</varname> were not reported by releases before
     14.)
     Note that
     <varname>server_version</varname>,
     <varname>server_encoding</varname> and
     <varname>integer_datetimes</varname>
     are pseudo-parameters that cannot change after startup.
     This set might change in the future, or even become configurable.
     Accordingly, a frontend should simply ignore ParameterStatus for
     parameters that it does not understand or care about.
    </para>

    <para>
     If a frontend issues a <command>LISTEN</command> command, then the
     backend will send a NotificationResponse message (not to be
     confused with NoticeResponse!)  whenever a
     <command>NOTIFY</command> command is executed for the same
     channel name.
    </para>

    <note>
     <para>
      At present, NotificationResponse can only be sent outside a
      transaction, and thus it will not occur in the middle of a
      command-response series, though it might occur just before ReadyForQuery.
      It is unwise to design frontend logic that assumes that, however.
      Good practice is to be able to accept NotificationResponse at any
      point in the protocol.
     </para>
    </note>
   </sect2>

   <sect2>
    <title>Canceling Requests in Progress</title>

    <para>
     During the processing of a query, the frontend might request
     cancellation of the query.  The cancel request is not sent
     directly on the open connection to the backend for reasons of
     implementation efficiency: we don't want to have the backend
     constantly checking for new input from the frontend during query
     processing.  Cancel requests should be relatively infrequent, so
     we make them slightly cumbersome in order to avoid a penalty in
     the normal case.
    </para>

    <para>
     To issue a cancel request, the frontend opens a new connection to
     the server and sends a CancelRequest message, rather than the
     StartupMessage message that would ordinarily be sent across a new
     connection.  The server will process this request and then close
     the connection.  For security reasons, no direct reply is made to
     the cancel request message.
    </para>

    <para>
     A CancelRequest message will be ignored unless it contains the
     same key data (PID and secret key) passed to the frontend during
     connection start-up.  If the request matches the PID and secret
     key for a currently executing backend, the processing of the
     current query is aborted.  (In the existing implementation, this is
     done by sending a special signal to the backend process that is
     processing the query.)
    </para>

    <para>
     The cancellation signal might or might not have any effect &mdash; for
     example, if it arrives after the backend has finished processing
     the query, then it will have no effect.  If the cancellation is
     effective, it results in the current command being terminated
     early with an error message.
    </para>

    <para>
     The upshot of all this is that for reasons of both security and
     efficiency, the frontend has no direct way to tell whether a
     cancel request has succeeded.  It must continue to wait for the
     backend to respond to the query.  Issuing a cancel simply improves
     the odds that the current query will finish soon, and improves the
     odds that it will fail with an error message instead of
     succeeding.
    </para>

    <para>
     Since the cancel request is sent across a new connection to the
     server and not across the regular frontend/backend communication
     link, it is possible for the cancel request to be issued by any
     process, not just the frontend whose query is to be canceled.
     This might provide additional flexibility when building
     multiple-process applications.  It also introduces a security
     risk, in that unauthorized persons might try to cancel queries.
     The security risk is addressed by requiring a dynamically
     generated secret key to be supplied in cancel requests.
    </para>
   </sect2>

   <sect2>
    <title>Termination</title>

    <para>
     The normal, graceful termination procedure is that the frontend
     sends a Terminate message and immediately closes the connection.
     On receipt of this message, the backend closes the connection and
     terminates.
    </para>

    <para>
     In rare cases (such as an administrator-commanded database shutdown)
     the backend might disconnect without any frontend request to do so.
     In such cases the backend will attempt to send an error or notice message
     giving the reason for the disconnection before it closes the connection.
    </para>

    <para>
     Other termination scenarios arise from various failure cases, such as core
     dump at one end or the other, loss of the communications link, loss of
     message-boundary synchronization, etc.  If either frontend or backend sees
     an unexpected closure of the connection, it should clean
     up and terminate.  The frontend has the option of launching a new backend
     by recontacting the server if it doesn't want to terminate itself.
     Closing the connection is also advisable if an unrecognizable message type
     is received, since this probably indicates loss of message-boundary sync.
    </para>

    <para>
     For either normal or abnormal termination, any open transaction is
     rolled back, not committed.  One should note however that if a
     frontend disconnects while a non-<command>SELECT</command> query
     is being processed, the backend will probably finish the query
     before noticing the disconnection.  If the query is outside any
     transaction block (<command>BEGIN</command> ... <command>COMMIT</command>
     sequence) then its results might be committed before the
     disconnection is recognized.
    </para>
   </sect2>

   <sect2>
    <title><acronym>SSL</acronym> Session Encryption</title>

    <para>
     If <productname>PostgreSQL</productname> was built with
     <acronym>SSL</acronym> support, frontend/backend communications
     can be encrypted using <acronym>SSL</acronym>.  This provides
     communication security in environments where attackers might be
     able to capture the session traffic. For more information on
     encrypting <productname>PostgreSQL</productname> sessions with
     <acronym>SSL</acronym>, see <xref linkend="ssl-tcp"/>.
    </para>

    <para>
     To initiate an <acronym>SSL</acronym>-encrypted connection, the
     frontend initially sends an SSLRequest message rather than a
     StartupMessage.  The server then responds with a single byte
     containing <literal>S</literal> or <literal>N</literal>, indicating that it is
     willing or unwilling to perform <acronym>SSL</acronym>,
     respectively.  The frontend might close the connection at this point
     if it is dissatisfied with the response.  To continue after
     <literal>S</literal>, perform an <acronym>SSL</acronym> startup handshake
     (not described here, part of the <acronym>SSL</acronym>
     specification) with the server.  If this is successful, continue
     with sending the usual StartupMessage.  In this case the
     StartupMessage and all subsequent data will be
     <acronym>SSL</acronym>-encrypted.  To continue after
     <literal>N</literal>, send the usual StartupMessage and proceed without
     encryption.
     (Alternatively, it is permissible to issue a GSSENCRequest message
     after an <literal>N</literal> response to try to
     use <acronym>GSSAPI</acronym> encryption instead
     of <acronym>SSL</acronym>.)
    </para>

    <para>
     The frontend should also be prepared to handle an ErrorMessage
     response to SSLRequest from the server.  This would only occur if
     the server predates the addition of <acronym>SSL</acronym> support
     to <productname>PostgreSQL</productname>.  (Such servers are now very ancient,
     and likely do not exist in the wild anymore.)
     In this case the connection must
     be closed, but the frontend might choose to open a fresh connection
     and proceed without requesting <acronym>SSL</acronym>.
    </para>

    <para>
     When <acronym>SSL</acronym> encryption can be performed, the server
     is expected to send only the single <literal>S</literal> byte and then
     wait for the frontend to initiate an <acronym>SSL</acronym> handshake.
     If additional bytes are available to read at this point, it likely
     means that a man-in-the-middle is attempting to perform a
     buffer-stuffing attack
     (<ulink url="https://www.postgresql.org/support/security/CVE-2021-23222/">CVE-2021-23222</ulink>).
     Frontends should be coded either to read exactly one byte from the
     socket before turning the socket over to their SSL library, or to
     treat it as a protocol violation if they find they have read additional
     bytes.
    </para>

    <para>
     An initial SSLRequest can also be used in a connection that is being
     opened to send a CancelRequest message.
    </para>

    <para>
     While the protocol itself does not provide a way for the server to
     force <acronym>SSL</acronym> encryption, the administrator can
     configure the server to reject unencrypted sessions as a byproduct
     of authentication checking.
    </para>
   </sect2>

   <sect2>
    <title><acronym>GSSAPI</acronym> Session Encryption</title>

    <para>
     If <productname>PostgreSQL</productname> was built with
     <acronym>GSSAPI</acronym> support, frontend/backend communications
     can be encrypted using <acronym>GSSAPI</acronym>.  This provides
     communication security in environments where attackers might be
     able to capture the session traffic. For more information on
     encrypting <productname>PostgreSQL</productname> sessions with
     <acronym>GSSAPI</acronym>, see <xref linkend="gssapi-enc"/>.
    </para>

    <para>
     To initiate a <acronym>GSSAPI</acronym>-encrypted connection, the
     frontend initially sends a GSSENCRequest message rather than a
     StartupMessage.  The server then responds with a single byte
     containing <literal>G</literal> or <literal>N</literal>, indicating that it
     is willing or unwilling to perform <acronym>GSSAPI</acronym> encryption,
     respectively.  The frontend might close the connection at this point
     if it is dissatisfied with the response.  To continue after
     <literal>G</literal>, using the GSSAPI C bindings as discussed in
     <ulink url="https://tools.ietf.org/html/rfc2744">RFC 2744</ulink>
     or equivalent, perform a <acronym>GSSAPI</acronym> initialization by
     calling <function>gss_init_sec_context()</function> in a loop and sending
     the result to the server, starting with an empty input and then with each
     result from the server, until it returns no output.  When sending the
     results of <function>gss_init_sec_context()</function> to the server,
     prepend the length of the message as a four byte integer in network byte
     order.
     To continue after
     <literal>N</literal>, send the usual StartupMessage and proceed without
     encryption.
     (Alternatively, it is permissible to issue an SSLRequest message
     after an <literal>N</literal> response to try to
     use <acronym>SSL</acronym> encryption instead
     of <acronym>GSSAPI</acronym>.)
    </para>

    <para>
     The frontend should also be prepared to handle an ErrorMessage
     response to GSSENCRequest from the server.  This would only occur if
     the server predates the addition of <acronym>GSSAPI</acronym> encryption
     support to <productname>PostgreSQL</productname>.  In this case the
     connection must be closed, but the frontend might choose to open a fresh
     connection and proceed without requesting <acronym>GSSAPI</acronym>
     encryption.
    </para>

    <para>
     When <acronym>GSSAPI</acronym> encryption can be performed, the server
     is expected to send only the single <literal>G</literal> byte and then
     wait for the frontend to initiate a <acronym>GSSAPI</acronym> handshake.
     If additional bytes are available to read at this point, it likely
     means that a man-in-the-middle is attempting to perform a
     buffer-stuffing attack
     (<ulink url="https://www.postgresql.org/support/security/CVE-2021-23222/">CVE-2021-23222</ulink>).
     Frontends should be coded either to read exactly one byte from the
     socket before turning the socket over to their GSSAPI library, or to
     treat it as a protocol violation if they find they have read additional
     bytes.
    </para>

    <para>
     An initial GSSENCRequest can also be used in a connection that is being
     opened to send a CancelRequest message.
    </para>

    <para>
     Once <acronym>GSSAPI</acronym> encryption has been successfully
     established, use <function>gss_wrap()</function> to
     encrypt the usual StartupMessage and all subsequent data, prepending the
     length of the result from <function>gss_wrap()</function> as a four byte
     integer in network byte order to the actual encrypted payload.  Note that
     the server will only accept encrypted packets from the client which are less
     than 16kB; <function>gss_wrap_size_limit()</function> should be used by the
     client to determine the size of the unencrypted message which will fit
     within this limit and larger messages should be broken up into multiple
     <function>gss_wrap()</function> calls.  Typical segments are 8kB of
     unencrypted data, resulting in encrypted packets of slightly larger than 8kB
     but well within the 16kB maximum.  The server can be expected to not send
     encrypted packets of larger than 16kB to the client.
    </para>

    <para>
     While the protocol itself does not provide a way for the server to
     force <acronym>GSSAPI</acronym> encryption, the administrator can
     configure the server to reject unencrypted sessions as a byproduct
     of authentication checking.
    </para>
   </sect2>
  </sect1>

 <sect1 id="sasl-authentication">
 <title>SASL Authentication</title>

 <para>
 <firstterm>SASL</firstterm> is a framework for authentication in connection-oriented
 protocols. At the moment, <productname>PostgreSQL</productname> implements two SASL
 authentication mechanisms, SCRAM-SHA-256 and SCRAM-SHA-256-PLUS. More
 might be added in the future. The below steps illustrate how SASL
 authentication is performed in general, while the next subsection gives
 more details on SCRAM-SHA-256 and SCRAM-SHA-256-PLUS.
 </para>

 <procedure>
 <title>SASL Authentication Message Flow</title>

 <step id="sasl-auth-begin">
 <para>
   To begin a SASL authentication exchange, the server sends an
   AuthenticationSASL message. It includes a list of SASL authentication
   mechanisms that the server can accept, in the server's preferred order.
 </para>
 </step>

 <step id="sasl-auth-initial-response">
 <para>
   The client selects one of the supported mechanisms from the list, and sends
   a SASLInitialResponse message to the server. The message includes the name
   of the selected mechanism, and an optional Initial Client Response, if the
   selected mechanism uses that.
 </para>
 </step>

 <step id="sasl-auth-continue">
 <para>
   One or more server-challenge and client-response message will follow. Each
   server-challenge is sent in an AuthenticationSASLContinue message, followed
   by a response from client in a SASLResponse message. The particulars of
   the messages are mechanism specific.
 </para>
 </step>

 <step id="sasl-auth-end">
 <para>
   Finally, when the authentication exchange is completed successfully, the
   server sends an AuthenticationSASLFinal message, followed
   immediately by an AuthenticationOk message. The AuthenticationSASLFinal
   contains additional server-to-client data, whose content is particular to the
   selected authentication mechanism. If the authentication mechanism doesn't
   use additional data that's sent at completion, the AuthenticationSASLFinal
   message is not sent.
 </para>
 </step>
 </procedure>

 <para>
 On error, the server can abort the authentication at any stage, and send an
 ErrorMessage.
 </para>

  <sect2 id="sasl-scram-sha-256">
   <title>SCRAM-SHA-256 Authentication</title>

   <para>
    The implemented SASL mechanisms at the moment
    are <literal>SCRAM-SHA-256</literal> and its variant with channel
    binding <literal>SCRAM-SHA-256-PLUS</literal>. They are described in
    detail in <ulink url="https://tools.ietf.org/html/rfc7677">RFC 7677</ulink>
    and <ulink url="https://tools.ietf.org/html/rfc5802">RFC 5802</ulink>.
   </para>

   <para>
 When SCRAM-SHA-256 is used in PostgreSQL, the server will ignore the user name
 that the client sends in the <structname>client-first-message</structname>. The user name
 that was already sent in the startup message is used instead.
 <productname>PostgreSQL</productname> supports multiple character encodings, while SCRAM
 dictates UTF-8 to be used for the user name, so it might be impossible to
 represent the PostgreSQL user name in UTF-8.
   </para>

   <para>
 The SCRAM specification dictates that the password is also in UTF-8, and is
 processed with the <firstterm>SASLprep</firstterm> algorithm.
 <productname>PostgreSQL</productname>, however, does not require UTF-8 to be used for
 the password. When a user's password is set, it is processed with SASLprep
 as if it was in UTF-8, regardless of the actual encoding used. However, if
 it is not a legal UTF-8 byte sequence, or it contains UTF-8 byte sequences
 that are prohibited by the SASLprep algorithm, the raw password will be used
 without SASLprep processing, instead of throwing an error. This allows the
 password to be normalized when it is in UTF-8, but still allows a non-UTF-8
 password to be used, and doesn't require the system to know which encoding
 the password is in.
   </para>

   <para>
 <firstterm>Channel binding</firstterm> is supported in PostgreSQL builds with
 SSL support. The SASL mechanism name for SCRAM with channel binding is
 <literal>SCRAM-SHA-256-PLUS</literal>.  The channel binding type used by
 PostgreSQL is <literal>tls-server-end-point</literal>.
   </para>

   <para>
    In <acronym>SCRAM</acronym> without channel binding, the server chooses
    a random number that is transmitted to the client to be mixed with the
    user-supplied password in the transmitted password hash.  While this
    prevents the password hash from being successfully retransmitted in
    a later session, it does not prevent a fake server between the real
    server and client from passing through the server's random value
    and successfully authenticating.
   </para>

   <para>
    <acronym>SCRAM</acronym> with channel binding prevents such
    man-in-the-middle attacks by mixing the signature of the server's
    certificate into the transmitted password hash. While a fake server can
    retransmit the real server's certificate, it doesn't have access to the
    private key matching that certificate, and therefore cannot prove it is
    the owner, causing SSL connection failure.
   </para>

 <procedure>
 <title>Example</title>
   <step id="scram-begin">
 <para>
   The server sends an AuthenticationSASL message. It includes a list of
   SASL authentication mechanisms that the server can accept.
   This will be <literal>SCRAM-SHA-256-PLUS</literal>
   and <literal>SCRAM-SHA-256</literal> if the server is built with SSL
   support, or else just the latter.
 </para>
 </step>
 <step id="scram-client-first">
 <para>
   The client responds by sending a SASLInitialResponse message, which
   indicates the chosen mechanism, <literal>SCRAM-SHA-256</literal> or
   <literal>SCRAM-SHA-256-PLUS</literal>. (A client is free to choose either
   mechanism, but for better security it should choose the channel-binding
   variant if it can support it.) In the Initial Client response field, the
   message contains the SCRAM <structname>client-first-message</structname>.
   The <structname>client-first-message</structname> also contains the channel
   binding type chosen by the client.
 </para>
 </step>
 <step id="scram-server-first">
 <para>
   Server sends an AuthenticationSASLContinue message, with a SCRAM
   <structname>server-first-message</structname> as the content.
 </para>
 </step>
 <step id="scram-client-final">
 <para>
   Client sends a SASLResponse message, with SCRAM
   <structname>client-final-message</structname> as the content.
 </para>
 </step>
 <step id="scram-server-final">
 <para>
   Server sends an AuthenticationSASLFinal message, with the SCRAM
   <structname>server-final-message</structname>, followed immediately by
   an AuthenticationOk message.
 </para>
 </step>
 </procedure>
 </sect2>
 </sect1>

 <sect1 id="protocol-replication">
 <title>Streaming Replication Protocol</title>

 <para>
 To initiate streaming replication, the frontend sends the
 <literal>replication</literal> parameter in the startup message. A Boolean
 value of <literal>true</literal> (or <literal>on</literal>,
 <literal>yes</literal>, <literal>1</literal>) tells the backend to go into
 physical replication walsender mode, wherein a small set of replication
 commands, shown below, can be issued instead of SQL statements.
 </para>

 <para>
 Passing <literal>database</literal> as the value for the
 <literal>replication</literal> parameter instructs the backend to go into
 logical replication walsender mode, connecting to the database specified in
 the <literal>dbname</literal> parameter.  In logical replication walsender
 mode, the replication commands shown below as well as normal SQL commands can
 be issued.
 </para>

 <para>
 In either physical replication or logical replication walsender mode, only the
 simple query protocol can be used.
 </para>

 <para>
  For the purpose of testing replication commands, you can make a replication
  connection via <application>psql</application> or any other
  <application>libpq</application>-using tool with a connection string including
  the <literal>replication</literal> option,
  e.g.:
 <programlisting>
 psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
 </programlisting>
  However, it is often more useful to use
  <xref linkend="app-pgreceivewal"/> (for physical replication) or
  <xref linkend="app-pgrecvlogical"/> (for logical replication).
 </para>

 <para>
 Replication commands are logged in the server log when
 <xref linkend="guc-log-replication-commands"/> is enabled.
 </para>

 <para>
 The commands accepted in replication mode are:
 <variablelist>
   <varlistentry>
     <term><literal>IDENTIFY_SYSTEM</literal>
      <indexterm><primary>IDENTIFY_SYSTEM</primary></indexterm>
     </term>
     <listitem>
      <para>
       Requests the server to identify itself. Server replies with a result
       set of a single row, containing four fields:
      </para>

      <para>
       <variablelist>
       <varlistentry>
       <term>
        <literal>systemid</literal> (<type>text</type>)
       </term>
       <listitem>
       <para>
        The unique system identifier identifying the cluster. This
        can be used to check that the base backup used to initialize the
        standby came from the same cluster.
       </para>
       </listitem>
       </varlistentry>

       <varlistentry>
       <term>
        <literal>timeline</literal> (<type>int4</type>)
       </term>
       <listitem>
       <para>
        Current timeline ID. Also useful to check that the standby is
        consistent with the primary.
       </para>
       </listitem>
       </varlistentry>

       <varlistentry>
       <term>
        <literal>xlogpos</literal> (<type>text</type>)
       </term>
       <listitem>
       <para>
        Current WAL flush location. Useful to get a known location in the
        write-ahead log where streaming can start.
       </para>
       </listitem>
       </varlistentry>

       <varlistentry>
       <term>
        <literal>dbname</literal> (<type>text</type>)
       </term>
       <listitem>
       <para>
        Database connected to or null.
       </para>
       </listitem>
       </varlistentry>

       </variablelist>
      </para>
     </listitem>
   </varlistentry>

   <varlistentry>
     <term><literal>SHOW</literal> <replaceable class="parameter">name</replaceable>
      <indexterm><primary>SHOW</primary></indexterm>
     </term>
     <listitem>
      <para>
       Requests the server to send the current setting of a run-time parameter.
       This is similar to the SQL command <xref linkend="sql-show"/>.
      </para>

      <variablelist>
       <varlistentry>
        <term><replaceable class="parameter">name</replaceable></term>
        <listitem>
          <para>
           The name of a run-time parameter. Available parameters are documented
           in <xref linkend="runtime-config"/>.
          </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
   </varlistentry>

   <varlistentry>
     <term><literal>TIMELINE_HISTORY</literal> <replaceable class="parameter">tli</replaceable>
      <indexterm><primary>TIMELINE_HISTORY</primary></indexterm>
     </term>
     <listitem>
      <para>
       Requests the server to send over the timeline history file for timeline
       <replaceable class="parameter">tli</replaceable>.  Server replies with a
       result set of a single row, containing two fields.  While the fields
       are labeled as <type>text</type>, they effectively return raw bytes,
       with no encoding conversion:
      </para>

      <para>
       <variablelist>
       <varlistentry>
       <term>
        <literal>filename</literal> (<type>text</type>)
       </term>
       <listitem>
       <para>
        File name of the timeline history file, e.g., <filename>00000002.history</filename>.
       </para>
       </listitem>
       </varlistentry>

       <varlistentry>
       <term>
        <literal>content</literal> (<type>text</type>)
       </term>
       <listitem>
       <para>
        Contents of the timeline history file.
       </para>
       </listitem>
       </varlistentry>

       </variablelist>
      </para>
     </listitem>
   </varlistentry>

   <varlistentry id="protocol-replication-create-slot" xreflabel="CREATE_REPLICATION_SLOT">
    <term><literal>CREATE_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</replaceable> [ <literal>TEMPORARY</literal> ] { <literal>PHYSICAL</literal> [ <literal>RESERVE_WAL</literal> ] | <literal>LOGICAL</literal> <replaceable class="parameter">output_plugin</replaceable> [ <literal>EXPORT_SNAPSHOT</literal> | <literal>NOEXPORT_SNAPSHOT</literal> | <literal>USE_SNAPSHOT</literal> ] }
      <indexterm><primary>CREATE_REPLICATION_SLOT</primary></indexterm>
     </term>
     <listitem>
      <para>
       Create a physical or logical replication
       slot. See <xref linkend="streaming-replication-slots"/> for more about
       replication slots.
      </para>
      <variablelist>
       <varlistentry>
        <term><replaceable class="parameter">slot_name</replaceable></term>
        <listitem>
          <para>
           The name of the slot to create. Must be a valid replication slot
           name (see <xref linkend="streaming-replication-slots-manipulation"/>).
          </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><replaceable class="parameter">output_plugin</replaceable></term>
        <listitem>
          <para>
           The name of the output plugin used for logical decoding
           (see <xref linkend="logicaldecoding-output-plugin"/>).
          </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>TEMPORARY</literal></term>
        <listitem>
         <para>
          Specify that this replication slot is a temporary one. Temporary
          slots are not saved to disk and are automatically dropped on error
          or when the session has finished.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>RESERVE_WAL</literal></term>
        <listitem>
         <para>
          Specify that this physical replication slot reserves <acronym>WAL</acronym>
          immediately.  Otherwise, <acronym>WAL</acronym> is only reserved upon
          connection from a streaming replication client.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>EXPORT_SNAPSHOT</literal></term>
        <term><literal>NOEXPORT_SNAPSHOT</literal></term>
        <term><literal>USE_SNAPSHOT</literal></term>
        <listitem>
         <para>
          Decides what to do with the snapshot created during logical slot
          initialization. <literal>EXPORT_SNAPSHOT</literal>, which is the default,
          will export the snapshot for use in other sessions. This option can't
          be used inside a transaction.  <literal>USE_SNAPSHOT</literal> will use the
          snapshot for the current transaction executing the command. This
          option must be used in a transaction, and
          <literal>CREATE_REPLICATION_SLOT</literal> must be the first command
          run in that transaction.  Finally, <literal>NOEXPORT_SNAPSHOT</literal> will
          just use the snapshot for logical decoding as normal but won't do
          anything else with it.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>

      <para>
       In response to this command, the server will send a one-row result set
       containing the following fields:

       <variablelist>
        <varlistentry>
         <term><literal>slot_name</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The name of the newly-created replication slot.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>consistent_point</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The WAL location at which the slot became consistent.  This is the
           earliest location from which streaming can start on this replication
           slot.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>snapshot_name</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The identifier of the snapshot exported by the command.  The
           snapshot is valid until a new command is executed on this connection
           or the replication connection is closed.  Null if the created slot
           is physical.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>output_plugin</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The name of the output plugin used by the newly-created replication
           slot.  Null if the created slot is physical.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>
     </listitem>
   </varlistentry>

   <varlistentry>
     <term><literal>START_REPLICATION</literal> [ <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable> ] [ <literal>PHYSICAL</literal> ] <replaceable class="parameter">XXX/XXX</replaceable> [ <literal>TIMELINE</literal> <replaceable class="parameter">tli</replaceable> ]
      <indexterm><primary>START_REPLICATION</primary></indexterm>
     </term>
     <listitem>
      <para>
       Instructs server to start streaming WAL, starting at
       WAL location <replaceable class="parameter">XXX/XXX</replaceable>.
       If <literal>TIMELINE</literal> option is specified,
       streaming starts on timeline <replaceable class="parameter">tli</replaceable>;
       otherwise, the server's current timeline is selected. The server can
       reply with an error, for example if the requested section of WAL has already
       been recycled. On success, the server responds with a CopyBothResponse
       message, and then starts to stream WAL to the frontend.
      </para>

      <para>
       If a slot's name is provided
       via <replaceable class="parameter">slot_name</replaceable>, it will be updated
       as replication progresses so that the server knows which WAL segments,
       and if <varname>hot_standby_feedback</varname> is on which transactions,
       are still needed by the standby.
      </para>

      <para>
       If the client requests a timeline that's not the latest but is part of
       the history of the server, the server will stream all the WAL on that
       timeline starting from the requested start point up to the point where
       the server switched to another timeline. If the client requests
       streaming at exactly the end of an old timeline, the server skips COPY
       mode entirely.
      </para>

      <para>
       After streaming all the WAL on a timeline that is not the latest one,
       the server will end streaming by exiting the COPY mode. When the client
       acknowledges this by also exiting COPY mode, the server sends a result
       set with one row and two columns, indicating the next timeline in this
       server's history. The first column is the next timeline's ID (type <type>int8</type>), and the
       second column is the WAL location where the switch happened (type <type>text</type>). Usually,
       the switch position is the end of the WAL that was streamed, but there
       are corner cases where the server can send some WAL from the old
       timeline that it has not itself replayed before promoting. Finally, the
       server sends two CommandComplete messages (one that ends the CopyData
       and the other ends the <literal>START_REPLICATION</literal> itself), and
       is ready to accept a new command.
      </para>

      <para>
       WAL data is sent as a series of CopyData messages.  (This allows
       other information to be intermixed; in particular the server can send
       an ErrorResponse message if it encounters a failure after beginning
       to stream.)  The payload of each CopyData message from server to the
       client contains a message of one of the following formats:
      </para>

      <para>
       <variablelist>
       <varlistentry>
       <term>
           XLogData (B)
       </term>
       <listitem>
       <para>
       <variablelist>
       <varlistentry>
       <term>
           Byte1('w')
       </term>
       <listitem>
       <para>
           Identifies the message as WAL data.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The starting point of the WAL data in this message.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The current end of WAL on the server.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The server's system clock at the time of transmission, as
           microseconds since midnight on 2000-01-01.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Byte<replaceable>n</replaceable>
       </term>
       <listitem>
       <para>
           A section of the WAL data stream.
       </para>
       <para>
           A single WAL record is never split across two XLogData messages.
           When a WAL record crosses a WAL page boundary, and is therefore
           already split using continuation records, it can be split at the page
           boundary. In other words, the first main WAL record and its
           continuation records can be sent in different XLogData messages.
       </para>
       </listitem>
       </varlistentry>
       </variablelist>
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Primary keepalive message (B)
       </term>
       <listitem>
       <para>
       <variablelist>
       <varlistentry>
       <term>
           Byte1('k')
       </term>
       <listitem>
       <para>
           Identifies the message as a sender keepalive.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The current end of WAL on the server.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The server's system clock at the time of transmission, as
           microseconds since midnight on 2000-01-01.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Byte1
       </term>
       <listitem>
       <para>
           1 means that the client should reply to this message as soon as
           possible, to avoid a timeout disconnect. 0 otherwise.
       </para>
       </listitem>
       </varlistentry>
       </variablelist>
       </para>
       </listitem>
       </varlistentry>
       </variablelist>
      </para>

      <para>
        The receiving process can send replies back to the sender at any time,
        using one of the following message formats (also in the payload of a
        CopyData message):
      </para>

      <para>
       <variablelist>
       <varlistentry>
       <term>
           Standby status update (F)
       </term>
       <listitem>
       <para>
       <variablelist>
       <varlistentry>
       <term>
           Byte1('r')
       </term>
       <listitem>
       <para>
           Identifies the message as a receiver status update.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The location of the last WAL byte + 1 received and written to disk
           in the standby.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The location of the last WAL byte + 1 flushed to disk in
           the standby.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The location of the last WAL byte + 1 applied in the standby.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The client's system clock at the time of transmission, as
           microseconds since midnight on 2000-01-01.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Byte1
       </term>
       <listitem>
       <para>
           If 1, the client requests the server to reply to this message
           immediately. This can be used to ping the server, to test if
           the connection is still healthy.
       </para>
       </listitem>
       </varlistentry>
       </variablelist>
       </para>
       </listitem>
       </varlistentry>
       </variablelist>
      </para>

      <para>
       <variablelist>
       <varlistentry>
       <term>
           Hot Standby feedback message (F)
       </term>
       <listitem>
       <para>
       <variablelist>
       <varlistentry>
       <term>
           Byte1('h')
       </term>
       <listitem>
       <para>
           Identifies the message as a Hot Standby feedback message.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int64
       </term>
       <listitem>
       <para>
           The client's system clock at the time of transmission, as
           microseconds since midnight on 2000-01-01.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int32
       </term>
       <listitem>
       <para>
           The standby's current global xmin, excluding the catalog_xmin from any
           replication slots. If both this value and the following
           catalog_xmin are 0 this is treated as a notification that Hot Standby
           feedback will no longer be sent on this connection. Later non-zero
           messages may reinitiate the feedback mechanism.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int32
       </term>
       <listitem>
       <para>
           The epoch of the global xmin xid on the standby.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int32
       </term>
       <listitem>
       <para>
           The lowest catalog_xmin of any replication slots on the standby. Set to 0
           if no catalog_xmin exists on the standby or if hot standby feedback is being
           disabled.
       </para>
       </listitem>
       </varlistentry>
       <varlistentry>
       <term>
           Int32
       </term>
       <listitem>
       <para>
           The epoch of the catalog_xmin xid on the standby.
       </para>
       </listitem>
       </varlistentry>
       </variablelist>
       </para>
       </listitem>
       </varlistentry>
       </variablelist>
      </para>
     </listitem>
   </varlistentry>
   <varlistentry>
     <term><literal>START_REPLICATION</literal> <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable> <literal>LOGICAL</literal> <replaceable class="parameter">XXX/XXX</replaceable> [ ( <replaceable>option_name</replaceable> [ <replaceable>option_value</replaceable> ] [, ...] ) ]</term>
     <listitem>
      <para>
       Instructs server to start streaming WAL for logical replication, starting
       at WAL location <replaceable class="parameter">XXX/XXX</replaceable>. The server can
       reply with an error, for example if the requested section of WAL has already
       been recycled. On success, server responds with a CopyBothResponse
       message, and then starts to stream WAL to the frontend.
      </para>

      <para>
       The messages inside the CopyBothResponse messages are of the same format
       documented for <literal>START_REPLICATION ... PHYSICAL</literal>, including
       two CommandComplete messages.
      </para>

      <para>
       The output plugin associated with the selected slot is used
       to process the output for streaming.
      </para>

      <variablelist>
       <varlistentry>
        <term><literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable></term>
        <listitem>
          <para>
           The name of the slot to stream changes from. This parameter is required,
           and must correspond to an existing logical replication slot created
           with <literal>CREATE_REPLICATION_SLOT</literal> in
           <literal>LOGICAL</literal> mode.
          </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term><replaceable class="parameter">XXX/XXX</replaceable></term>
        <listitem>
         <para>
          The WAL location to begin streaming at.
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term><replaceable class="parameter">option_name</replaceable></term>
        <listitem>
         <para>
          The name of an option passed to the slot's logical decoding plugin.
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term><replaceable class="parameter">option_value</replaceable></term>
        <listitem>
         <para>
          Optional value, in the form of a string constant, associated with the
          specified option.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
   </varlistentry>

   <varlistentry>
     <term>
      <literal>DROP_REPLICATION_SLOT</literal> <replaceable class="parameter">slot_name</replaceable> <optional> <literal>WAIT</literal> </optional>
      <indexterm><primary>DROP_REPLICATION_SLOT</primary></indexterm>
     </term>
     <listitem>
      <para>
       Drops a replication slot, freeing any reserved server-side resources.
       If the slot is a logical slot that was created in a database other than
       the database the walsender is connected to, this command fails.
      </para>
      <variablelist>
       <varlistentry>
        <term><replaceable class="parameter">slot_name</replaceable></term>
        <listitem>
          <para>
           The name of the slot to drop.
          </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>WAIT</literal></term>
        <listitem>
         <para>
          This option causes the command to wait if the slot is active until
          it becomes inactive, instead of the default behavior of raising an
          error.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
   </varlistentry>

   <varlistentry id="protocol-replication-base-backup" xreflabel="BASE_BACKUP">
     <term><literal>BASE_BACKUP</literal> [ <literal>LABEL</literal> <replaceable>'label'</replaceable> ] [ <literal>PROGRESS</literal> ] [ <literal>FAST</literal> ] [ <literal>WAL</literal> ] [ <literal>NOWAIT</literal> ] [ <literal>MAX_RATE</literal> <replaceable>rate</replaceable> ] [ <literal>TABLESPACE_MAP</literal> ] [ <literal>NOVERIFY_CHECKSUMS</literal> ] [ <literal>MANIFEST</literal> <replaceable>manifest_option</replaceable> ] [ <literal>MANIFEST_CHECKSUMS</literal> <replaceable>checksum_algorithm</replaceable> ]
      <indexterm><primary>BASE_BACKUP</primary></indexterm>
     </term>
     <listitem>
      <para>
       Instructs the server to start streaming a base backup.
       The system will automatically be put in backup mode before the backup
       is started, and taken out of it when the backup is complete. The
       following options are accepted:
       <variablelist>
        <varlistentry>
         <term><literal>LABEL</literal> <replaceable>'label'</replaceable></term>
         <listitem>
          <para>
           Sets the label of the backup. If none is specified, a backup label
           of <literal>base backup</literal> will be used. The quoting rules
           for the label are the same as a standard SQL string with
           <xref linkend="guc-standard-conforming-strings"/> turned on.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>PROGRESS</literal></term>
         <listitem>
          <para>
           Request information required to generate a progress report. This will
           send back an approximate size in the header of each tablespace, which
           can be used to calculate how far along the stream is done. This is
           calculated by enumerating all the file sizes once before the transfer
           is even started, and might as such have a negative impact on the
           performance.  In particular, it might take longer before the first data
           is streamed. Since the database files can change during the backup,
           the size is only approximate and might both grow and shrink between
           the time of approximation and the sending of the actual files.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>FAST</literal></term>
         <listitem>
          <para>
           Request a fast checkpoint.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>WAL</literal></term>
         <listitem>
          <para>
           Include the necessary WAL segments in the backup. This will include
           all the files between start and stop backup in the
           <filename>pg_wal</filename> directory of the base directory tar
           file.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>NOWAIT</literal></term>
         <listitem>
          <para>
           By default, the backup will wait until the last required WAL
           segment has been archived, or emit a warning if log archiving is
           not enabled. Specifying <literal>NOWAIT</literal> disables both
           the waiting and the warning, leaving the client responsible for
           ensuring the required log is available.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>MAX_RATE</literal> <replaceable>rate</replaceable></term>
         <listitem>
          <para>
           Limit (throttle) the maximum amount of data transferred from server
           to client per unit of time.  The expected unit is kilobytes per second.
           If this option is specified, the value must either be equal to zero
           or it must fall within the range from 32 kB through 1 GB (inclusive).
           If zero is passed or the option is not specified, no restriction is
           imposed on the transfer.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>TABLESPACE_MAP</literal></term>
         <listitem>
          <para>
           Include information about symbolic links present in the directory
           <filename>pg_tblspc</filename> in a file named
           <filename>tablespace_map</filename>. The tablespace map file includes
           each symbolic link name as it exists in the directory
           <filename>pg_tblspc/</filename> and the full path of that symbolic link.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>NOVERIFY_CHECKSUMS</literal></term>
         <listitem>
          <para>
           By default, checksums are verified during a base backup if they are
           enabled. Specifying <literal>NOVERIFY_CHECKSUMS</literal> disables
           this verification.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>MANIFEST</literal> <replaceable>manifest_option</replaceable></term>
         <listitem>
          <para>
           When this option is specified with a value of <literal>yes</literal>
           or <literal>force-encode</literal>, a backup manifest is created
           and sent along with the backup.  The manifest is a list of every
           file present in the backup with the exception of any WAL files that
           may be included. It also stores the size, last modification time, and
           optionally a checksum for each file.
           A value of <literal>force-encode</literal> forces all filenames
           to be hex-encoded; otherwise, this type of encoding is performed only
           for files whose names are non-UTF8 octet sequences.
           <literal>force-encode</literal> is intended primarily for testing
           purposes, to be sure that clients which read the backup manifest
           can handle this case. For compatibility with previous releases,
           the default is <literal>MANIFEST 'no'</literal>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>MANIFEST_CHECKSUMS</literal> <replaceable>checksum_algorithm</replaceable></term>
         <listitem>
          <para>
           Specifies the checksum algorithm that should be applied to each file included
           in the backup manifest. Currently, the available
           algorithms are <literal>NONE</literal>, <literal>CRC32C</literal>,
           <literal>SHA224</literal>, <literal>SHA256</literal>,
           <literal>SHA384</literal>, and <literal>SHA512</literal>.
           The default is <literal>CRC32C</literal>.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>
      <para>
       When the backup is started, the server will first send two
       ordinary result sets, followed by one or more CopyOutResponse
       results.
      </para>
      <para>
       The first ordinary result set contains the starting position of the
       backup, in a single row with two columns. The first column contains
       the start position given in XLogRecPtr format, and the second column
       contains the corresponding timeline ID.
      </para>
      <para>
       The second ordinary result set has one row for each tablespace.
       The fields in this row are:
       <variablelist>
        <varlistentry>
         <term><literal>spcoid</literal> (<type>oid</type>)</term>
         <listitem>
          <para>
           The OID of the tablespace, or null if it's the base
           directory.
          </para>
         </listitem>
        </varlistentry>
        <varlistentry>
         <term><literal>spclocation</literal> (<type>text</type>)</term>
         <listitem>
          <para>
           The full path of the tablespace directory, or null
           if it's the base directory.
          </para>
         </listitem>
        </varlistentry>
        <varlistentry>
         <term><literal>size</literal> (<type>int8</type>)</term>
         <listitem>
          <para>
           The approximate size of the tablespace, in kilobytes (1024 bytes),
           if progress report has been requested; otherwise it's null.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </para>
      <para>
       After the second regular result set, one or more CopyOutResponse results
       will be sent, one for the main data directory and one for each additional tablespace other
       than <literal>pg_default</literal> and <literal>pg_global</literal>. The data in
       the CopyOutResponse results will be a tar format (following the
       <quote>ustar interchange format</quote> specified in the POSIX 1003.1-2008
       standard) dump of the tablespace contents, except that the two trailing
       blocks of zeroes specified in the standard are omitted.
       After the tar data is complete, and if a backup manifest was requested,
       another CopyOutResponse result is sent, containing the manifest data for the
       current base backup. In any case, a final ordinary result set will be
       sent, containing the WAL end position of the backup, in the same format as
       the start position.
      </para>

      <para>
       The tar archive for the data directory and each tablespace will contain
       all files in the directories, regardless of whether they are
       <productname>PostgreSQL</productname> files or other files added to the same
       directory. The only excluded files are:
       <itemizedlist spacing="compact" mark="bullet">
        <listitem>
         <para>
          <filename>postmaster.pid</filename>
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>postmaster.opts</filename>
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>pg_internal.init</filename> (found in multiple directories)
         </para>
        </listitem>
        <listitem>
         <para>
          Various temporary files and directories created during the operation
          of the PostgreSQL server, such as any file or directory beginning
          with <filename>pgsql_tmp</filename> and temporary relations.
         </para>
        </listitem>
        <listitem>
         <para>
          Unlogged relations, except for the init fork which is required to
          recreate the (empty) unlogged relation on recovery.
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>pg_wal</filename>, including subdirectories. If the backup is run
          with WAL files included, a synthesized version of <filename>pg_wal</filename> will be
          included, but it will only contain the files necessary for the
          backup to work, not the rest of the contents.
         </para>
        </listitem>
        <listitem>
         <para>
          <filename>pg_dynshmem</filename>, <filename>pg_notify</filename>,
          <filename>pg_replslot</filename>, <filename>pg_serial</filename>,
          <filename>pg_snapshots</filename>, <filename>pg_stat_tmp</filename>, and
          <filename>pg_subtrans</filename> are copied as empty directories (even if
          they are symbolic links).
         </para>
        </listitem>
        <listitem>
         <para>
          Files other than regular files and directories, such as symbolic
          links (other than for the directories listed above) and special
          device files, are skipped.  (Symbolic links
          in <filename>pg_tblspc</filename> are maintained.)
         </para>
        </listitem>
       </itemizedlist>
       Owner, group, and file mode are set if the underlying file system on
       the server supports it.
      </para>
     </listitem>
   </varlistentry>
 </variablelist>

 </para>

 </sect1>

 <sect1 id="protocol-logical-replication">
  <title>Logical Streaming Replication Protocol</title>

  <para>
   This section describes the logical replication protocol, which is the message
   flow started by the <literal>START_REPLICATION</literal>
   <literal>SLOT</literal> <replaceable class="parameter">slot_name</replaceable>
   <literal>LOGICAL</literal> replication command.
  </para>

  <para>
   The logical streaming replication protocol builds on the primitives of
   the physical streaming replication protocol.
  </para>

  <sect2 id="protocol-logical-replication-params">
   <title>Logical Streaming Replication Parameters</title>

   <para>
    The logical replication <literal>START_REPLICATION</literal> command
    accepts following parameters:

    <variablelist>
     <varlistentry>
      <term>
       proto_version
      </term>
      <listitem>
       <para>
        Protocol version. Currently versions <literal>1</literal> and
        <literal>2</literal> are supported. The version <literal>2</literal>
        is supported only for server version 14 and above, and it allows
        streaming of large in-progress transactions.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       publication_names
      </term>
      <listitem>
       <para>
        Comma separated list of publication names for which to subscribe
        (receive changes). The individual publication names are treated
        as standard objects names and can be quoted the same as needed.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

   </para>
  </sect2>

  <sect2 id="protocol-logical-messages">
   <title>Logical Replication Protocol Messages</title>

   <para>
    The individual protocol messages are discussed in the following
    subsections. Individual messages are described in
    <xref linkend="protocol-logicalrep-message-formats"/>.
   </para>

   <para>
    All top-level protocol messages begin with a message type byte.
    While represented in code as a character, this is a signed byte with no
    associated encoding.
   </para>

   <para>
    Since the streaming replication protocol supplies a message length there
    is no need for top-level protocol messages to embed a length in their
    header.
   </para>

  </sect2>

  <sect2 id="protocol-logical-messages-flow">
   <title>Logical Replication Protocol Message Flow</title>

   <para>
    With the exception of the <literal>START_REPLICATION</literal> command and
    the replay progress messages, all information flows only from the backend
    to the frontend.
   </para>

   <para>
    The logical replication protocol sends individual transactions one by one.
    This means that all messages between a pair of Begin and Commit messages
    belong to the same transaction. It also sends changes of large in-progress
    transactions between a pair of Stream Start and Stream Stop messages. The
    last stream of such a transaction contains Stream Commit or Stream Abort
    message.
   </para>

   <para>
    Every sent transaction contains zero or more DML messages (Insert,
    Update, Delete). In case of a cascaded setup it can also contain Origin
    messages. The origin message indicates that the transaction originated on
    different replication node. Since a replication node in the scope of logical
    replication protocol can be pretty much anything, the only identifier
    is the origin name. It's downstream's responsibility to handle this as
    needed (if needed). The Origin message is always sent before any DML
    messages in the transaction.
   </para>

   <para>
    Every DML message contains a relation OID, identifying the publisher's
    relation that was acted on.  Before the first DML message for a given
    relation OID, a Relation message will be sent, describing the schema of
    that relation.  Subsequently, a new Relation message will be sent if
    the relation's definition has changed since the last Relation message
    was sent for it.  (The protocol assumes that the client is capable of
    remembering this metadata for as many relations as needed.)
   </para>

   <para>
    Relation messages identify column types by their OIDs.  In the case
    of a built-in type, it is assumed that the client can look up that
    type OID locally, so no additional data is needed.  For a non-built-in
    type OID, a Type message will be sent before the Relation message,
    to provide the type name associated with that OID.  Thus, a client that
    needs to specifically identify the types of relation columns should
    cache the contents of Type messages, and first consult that cache to
    see if the type OID is defined there.  If not, look up the type OID
    locally.
   </para>
  </sect2>
 </sect1>

 <sect1 id="protocol-message-types">
 <title>Message Data Types</title>

 <para>
 This section describes the base data types used in messages.

 <variablelist>

 <varlistentry>
 <term>
         Int<replaceable>n</replaceable>(<replaceable>i</replaceable>)
 </term>
 <listitem>
 <para>
                 An <replaceable>n</replaceable>-bit integer in network byte
                 order (most significant byte first).
                 If <replaceable>i</replaceable> is specified it
                 is the exact value that will appear, otherwise the value
                 is variable.  Eg. Int16, Int32(42).
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Int<replaceable>n</replaceable>[<replaceable>k</replaceable>]
 </term>
 <listitem>
 <para>
                 An array of <replaceable>k</replaceable>
                 <replaceable>n</replaceable>-bit integers, each in network
                 byte order.  The array length <replaceable>k</replaceable>
                 is always determined by an earlier field in the message.
                 Eg. Int16[M].
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         String(<replaceable>s</replaceable>)
 </term>
 <listitem>
 <para>
                 A null-terminated string (C-style string).  There is no
                 specific length limitation on strings.
                 If <replaceable>s</replaceable> is specified it is the exact
                 value that will appear, otherwise the value is variable.
                 Eg. String, String("user").
 </para>

 <note>
 <para>
 <emphasis>There is no predefined limit</emphasis> on the length of a string
 that can be returned by the backend.  Good coding strategy for a frontend
 is to use an expandable buffer so that anything that fits in memory can be
 accepted.  If that's not feasible, read the full string and discard trailing
 characters that don't fit into your fixed-size buffer.
 </para>
 </note>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>(<replaceable>c</replaceable>)
 </term>
 <listitem>
 <para>
                 Exactly <replaceable>n</replaceable> bytes.  If the field
                 width <replaceable>n</replaceable> is not a constant, it is
                 always determinable from an earlier field in the message.
                 If <replaceable>c</replaceable> is specified it is the exact
                 value.  Eg. Byte2, Byte1('\n').
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </sect1>

 <sect1 id="protocol-message-formats">
 <title>Message Formats</title>

 <para>
 This section describes the detailed format of each message.  Each is marked to
 indicate that it can be sent by a frontend (F), a backend (B), or both
 (F &amp; B).
 Notice that although each message includes a byte count at the beginning,
 the message format is defined so that the message end can be found without
 reference to the byte count.  This aids validity checking.  (The CopyData
 message is an exception, because it forms part of a data stream; the contents
 of any individual CopyData message cannot be interpretable on their own.)
 </para>

 <variablelist>


 <varlistentry>
 <term>
 AuthenticationOk (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(0)
 </term>
 <listitem>
 <para>
                 Specifies that the authentication was successful.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationKerberosV5 (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(2)
 </term>
 <listitem>
 <para>
                 Specifies that Kerberos V5 authentication is required.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationCleartextPassword (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(3)
 </term>
 <listitem>
 <para>
                 Specifies that a clear-text password is required.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationMD5Password (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(12)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(5)
 </term>
 <listitem>
 <para>
                 Specifies that an MD5-encrypted password is required.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte4
 </term>
 <listitem>
 <para>
                 The salt to use when encrypting the password.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationSCMCredential (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(6)
 </term>
 <listitem>
 <para>
                 Specifies that an SCM credentials message is required.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationGSS (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(7)
 </term>
 <listitem>
 <para>
                 Specifies that GSSAPI authentication is required.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationGSSContinue (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Specifies that this message contains GSSAPI or SSPI data.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 GSSAPI or SSPI authentication data.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationSSPI (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(9)
 </term>
 <listitem>
 <para>
                 Specifies that SSPI authentication is required.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationSASL (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(10)
 </term>
 <listitem>
 <para>
                 Specifies that SASL authentication is required.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 The message body is a list of SASL authentication mechanisms, in the
 server's order of preference. A zero byte is required as terminator after
 the last authentication mechanism name. For each mechanism, there is the
 following:
 <variablelist>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Name of a SASL authentication mechanism.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationSASLContinue (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(11)
 </term>
 <listitem>
 <para>
                 Specifies that this message contains a SASL challenge.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 SASL data, specific to the SASL mechanism being used.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 AuthenticationSASLFinal (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as an authentication request.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(12)
 </term>
 <listitem>
 <para>
                 Specifies that SASL authentication has completed.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 SASL outcome "additional data", specific to the SASL mechanism
                 being used.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 BackendKeyData (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('K')
 </term>
 <listitem>
 <para>
                 Identifies the message as cancellation key data.
                 The frontend must save these values if it wishes to be
                 able to issue CancelRequest messages later.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(12)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The process ID of this backend.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The secret key of this backend.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Bind (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('B')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Bind command.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the destination portal
                 (an empty string selects the unnamed portal).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the source prepared statement
                 (an empty string selects the unnamed prepared statement).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of parameter format codes that follow
                 (denoted <replaceable>C</replaceable> below).
                 This can be zero to indicate that there are no parameters
                 or that the parameters all use the default format (text);
                 or one, in which case the specified format code is applied
                 to all parameters; or it can equal the actual number of
                 parameters.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16[<replaceable>C</replaceable>]
 </term>
 <listitem>
 <para>
                 The parameter format codes.  Each must presently be
                 zero (text) or one (binary).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of parameter values that follow (possibly zero).
                 This must match the number of parameters needed by the query.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Next, the following pair of fields appear for each parameter:
 <variablelist>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The length of the parameter value, in bytes (this count
                 does not include itself).  Can be zero.
                 As a special case, -1 indicates a NULL parameter value.
                 No value bytes follow in the NULL case.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 The value of the parameter, in the format indicated by the
                 associated format code.
                 <replaceable>n</replaceable> is the above length.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         After the last parameter, the following fields appear:
 <variablelist>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of result-column format codes that follow
                 (denoted <replaceable>R</replaceable> below).
                 This can be zero to indicate that there are no result columns
                 or that the result columns should all use the default format
                 (text);
                 or one, in which case the specified format code is applied
                 to all result columns (if any); or it can equal the actual
                 number of result columns of the query.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16[<replaceable>R</replaceable>]
 </term>
 <listitem>
 <para>
                 The result-column format codes.  Each must presently be
                 zero (text) or one (binary).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 BindComplete (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('2')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Bind-complete indicator.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CancelRequest (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Int32(16)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(80877102)
 </term>
 <listitem>
 <para>
                 The cancel request code.  The value is chosen to contain
                 <literal>1234</literal> in the most significant 16 bits, and <literal>5678</literal> in the
                 least significant 16 bits.  (To avoid confusion, this code
                 must not be the same as any protocol version number.)
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The process ID of the target backend.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The secret key for the target backend.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Close (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('C')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Close command.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte1
 </term>
 <listitem>
 <para>
                 '<literal>S</literal>' to close a prepared statement; or
                 '<literal>P</literal>' to close a portal.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the prepared statement or portal to close
                 (an empty string selects the unnamed prepared statement
                 or portal).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CloseComplete (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('3')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Close-complete indicator.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CommandComplete (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('C')
 </term>
 <listitem>
 <para>
                 Identifies the message as a command-completed response.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
        <para>
         The command tag.  This is usually a single
         word that identifies which SQL command was completed.
        </para>

        <para>
         For an <command>INSERT</command> command, the tag is
         <literal>INSERT <replaceable>oid</replaceable>
         <replaceable>rows</replaceable></literal>, where
         <replaceable>rows</replaceable> is the number of rows
         inserted. <replaceable>oid</replaceable> used to be the object ID
         of the inserted row if <replaceable>rows</replaceable> was 1
         and the target table had OIDs, but OIDs system columns are
         not supported anymore; therefore <replaceable>oid</replaceable>
         is always 0.
        </para>

        <para>
         For a <command>DELETE</command> command, the tag is
         <literal>DELETE <replaceable>rows</replaceable></literal> where
         <replaceable>rows</replaceable> is the number of rows deleted.
        </para>

        <para>
         For an <command>UPDATE</command> command, the tag is
         <literal>UPDATE <replaceable>rows</replaceable></literal> where
         <replaceable>rows</replaceable> is the number of rows updated.
        </para>

        <para>
         For a <command>SELECT</command> or <command>CREATE TABLE AS</command>
         command, the tag is <literal>SELECT <replaceable>rows</replaceable></literal>
         where <replaceable>rows</replaceable> is the number of rows retrieved.
        </para>

        <para>
         For a <command>MOVE</command> command, the tag is
         <literal>MOVE <replaceable>rows</replaceable></literal> where
         <replaceable>rows</replaceable> is the number of rows the
         cursor's position has been changed by.
        </para>

        <para>
         For a <command>FETCH</command> command, the tag is
         <literal>FETCH <replaceable>rows</replaceable></literal> where
         <replaceable>rows</replaceable> is the number of rows that
         have been retrieved from the cursor.
        </para>

        <para>
         For a <command>COPY</command> command, the tag is
         <literal>COPY <replaceable>rows</replaceable></literal> where
         <replaceable>rows</replaceable> is the number of rows copied.
         (Note: the row count appears only in
         <productname>PostgreSQL</productname> 8.2 and later.)
        </para>

 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CopyData (F &amp; B)
 </term>
 <listitem>
 <para>
 <variablelist>
 <varlistentry>
 <term>
         Byte1('d')
 </term>
 <listitem>
 <para>
                 Identifies the message as <command>COPY</command> data.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 Data that forms part of a <command>COPY</command> data stream.  Messages sent
                 from the backend will always correspond to single data rows,
                 but messages sent by frontends might divide the data stream
                 arbitrarily.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CopyDone (F &amp; B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('c')
 </term>
 <listitem>
 <para>
                 Identifies the message as a <command>COPY</command>-complete indicator.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CopyFail (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('f')
 </term>
 <listitem>
 <para>
                 Identifies the message as a <command>COPY</command>-failure indicator.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 An error message to report as the cause of failure.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CopyInResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('G')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Start Copy In response.
                 The frontend must now send copy-in data (if not
                 prepared to do so, send a CopyFail message).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 0 indicates the overall <command>COPY</command> format is textual (rows
                 separated by newlines, columns separated by separator
                 characters, etc).
                 1 indicates the overall copy format is binary (similar
                 to DataRow format).
                 See <xref linkend="sql-copy"/>
                 for more information.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of columns in the data to be copied
                 (denoted <replaceable>N</replaceable> below).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16[<replaceable>N</replaceable>]
 </term>
 <listitem>
 <para>
                 The format codes to be used for each column.
                 Each must presently be zero (text) or one (binary).
                 All must be zero if the overall copy format is textual.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CopyOutResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('H')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Start Copy Out response.
                 This message will be followed by copy-out data.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 0 indicates the overall <command>COPY</command> format
                 is textual (rows separated by newlines, columns
                 separated by separator characters, etc). 1 indicates
                 the overall copy format is binary (similar to DataRow
                 format). See <xref linkend="sql-copy"/> for more information.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of columns in the data to be copied
                 (denoted <replaceable>N</replaceable> below).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16[<replaceable>N</replaceable>]
 </term>
 <listitem>
 <para>
                 The format codes to be used for each column.
                 Each must presently be zero (text) or one (binary).
                 All must be zero if the overall copy format is textual.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 CopyBothResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('W')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Start Copy Both response.
                 This message is used only for Streaming Replication.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 0 indicates the overall <command>COPY</command> format
                 is textual (rows separated by newlines, columns
                 separated by separator characters, etc). 1 indicates
                 the overall copy format is binary (similar to DataRow
                 format). See <xref linkend="sql-copy"/> for more information.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of columns in the data to be copied
                 (denoted <replaceable>N</replaceable> below).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16[<replaceable>N</replaceable>]
 </term>
 <listitem>
 <para>
                 The format codes to be used for each column.
                 Each must presently be zero (text) or one (binary).
                 All must be zero if the overall copy format is textual.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 DataRow (B)
 </term>
 <listitem>
 <para>
 <variablelist>
 <varlistentry>
 <term>
         Byte1('D')
 </term>
 <listitem>
 <para>
                 Identifies the message as a data row.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of column values that follow (possibly zero).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Next, the following pair of fields appear for each column:
 <variablelist>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The length of the column value, in bytes (this count
                 does not include itself).  Can be zero.
                 As a special case, -1 indicates a NULL column value.
                 No value bytes follow in the NULL case.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 The value of the column, in the format indicated by the
                 associated format code.
                 <replaceable>n</replaceable> is the above length.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Describe (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('D')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Describe command.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte1
 </term>
 <listitem>
 <para>
                 '<literal>S</literal>' to describe a prepared statement; or
                 '<literal>P</literal>' to describe a portal.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the prepared statement or portal to describe
                 (an empty string selects the unnamed prepared statement
                 or portal).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 EmptyQueryResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('I')
 </term>
 <listitem>
 <para>
                 Identifies the message as a response to an empty query string.
                 (This substitutes for CommandComplete.)
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 ErrorResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('E')
 </term>
 <listitem>
 <para>
                 Identifies the message as an error.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         The message body consists of one or more identified fields,
         followed by a zero byte as a terminator.  Fields can appear in
         any order.  For each field there is the following:
 <variablelist>
 <varlistentry>
 <term>
         Byte1
 </term>
 <listitem>
 <para>
                 A code identifying the field type; if zero, this is
                 the message terminator and no string follows.
                 The presently defined field types are listed in
                 <xref linkend="protocol-error-fields"/>.
                 Since more field types might be added in future,
                 frontends should silently ignore fields of unrecognized
                 type.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The field value.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Execute (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('E')
 </term>
 <listitem>
 <para>
                 Identifies the message as an Execute command.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the portal to execute
                 (an empty string selects the unnamed portal).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Maximum number of rows to return, if portal contains
                 a query that returns rows (ignored otherwise).  Zero
                 denotes <quote>no limit</quote>.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Flush (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('H')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Flush command.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 FunctionCall (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('F')
 </term>
 <listitem>
 <para>
                 Identifies the message as a function call.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Specifies the object ID of the function to call.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of argument format codes that follow
                 (denoted <replaceable>C</replaceable> below).
                 This can be zero to indicate that there are no arguments
                 or that the arguments all use the default format (text);
                 or one, in which case the specified format code is applied
                 to all arguments; or it can equal the actual number of
                 arguments.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16[<replaceable>C</replaceable>]
 </term>
 <listitem>
 <para>
                 The argument format codes.  Each must presently be
                 zero (text) or one (binary).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 Specifies the number of arguments being supplied to the
                 function.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Next, the following pair of fields appear for each argument:
 <variablelist>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The length of the argument value, in bytes (this count
                 does not include itself).  Can be zero.
                 As a special case, -1 indicates a NULL argument value.
                 No value bytes follow in the NULL case.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 The value of the argument, in the format indicated by the
                 associated format code.
                 <replaceable>n</replaceable> is the above length.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         After the last argument, the following field appears:
 <variablelist>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The format code for the function result. Must presently be
                 zero (text) or one (binary).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 FunctionCallResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('V')
 </term>
 <listitem>
 <para>
                 Identifies the message as a function call result.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The length of the function result value, in bytes (this count
                 does not include itself).  Can be zero.
                 As a special case, -1 indicates a NULL function result.
                 No value bytes follow in the NULL case.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 The value of the function result, in the format indicated by
                 the associated format code.
                 <replaceable>n</replaceable> is the above length.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 GSSENCRequest (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(80877104)
 </term>
 <listitem>
 <para>
                 The <acronym>GSSAPI</acronym> Encryption request code.  The value is chosen to contain
                 <literal>1234</literal> in the most significant 16 bits, and <literal>5680</literal> in the
                 least significant 16 bits.  (To avoid confusion, this code
                 must not be the same as any protocol version number.)
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 GSSResponse (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('p')
 </term>
 <listitem>
 <para>
                 Identifies the message as a GSSAPI or SSPI response. Note that
                 this is also used for SASL and password response messages.
                 The exact message type can be deduced from the context.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 GSSAPI/SSPI specific message data.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 NegotiateProtocolVersion (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('v')
 </term>
 <listitem>
 <para>
                 Identifies the message as a protocol version negotiation
                 message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Newest minor protocol version supported by the server
                 for the major protocol version requested by the client.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Number of protocol options not recognized by the server.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Then, for protocol option not recognized by the server, there
         is the following:
 <variablelist>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The option name.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 NoData (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('n')
 </term>
 <listitem>
 <para>
                 Identifies the message as a no-data indicator.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 NoticeResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('N')
 </term>
 <listitem>
 <para>
                 Identifies the message as a notice.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         The message body consists of one or more identified fields,
         followed by a zero byte as a terminator.  Fields can appear in
         any order.  For each field there is the following:
 <variablelist>
 <varlistentry>
 <term>
         Byte1
 </term>
 <listitem>
 <para>
                 A code identifying the field type; if zero, this is
                 the message terminator and no string follows.
                 The presently defined field types are listed in
                 <xref linkend="protocol-error-fields"/>.
                 Since more field types might be added in future,
                 frontends should silently ignore fields of unrecognized
                 type.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The field value.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 NotificationResponse (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('A')
 </term>
 <listitem>
 <para>
                 Identifies the message as a notification response.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The process ID of the notifying backend process.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the channel that the notify has been raised on.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The <quote>payload</quote> string passed from the notifying process.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 ParameterDescription (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('t')
 </term>
 <listitem>
 <para>
                 Identifies the message as a parameter description.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of parameters used by the statement
                 (can be zero).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Then, for each parameter, there is the following:
 <variablelist>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Specifies the object ID of the parameter data type.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 ParameterStatus (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('S')
 </term>
 <listitem>
 <para>
                 Identifies the message as a run-time parameter status report.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the run-time parameter being reported.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The current value of the parameter.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Parse (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('P')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Parse command.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The name of the destination prepared statement
                 (an empty string selects the unnamed prepared statement).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The query string to be parsed.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The number of parameter data types specified
                 (can be zero).  Note that this is not an indication of
                 the number of parameters that might appear in the
                 query string, only the number that the frontend wants to
                 prespecify types for.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Then, for each parameter, there is the following:
 <variablelist>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Specifies the object ID of the parameter data type.
                 Placing a zero here is equivalent to leaving the type
                 unspecified.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 ParseComplete (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('1')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Parse-complete indicator.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 PasswordMessage (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('p')
 </term>
 <listitem>
 <para>
                 Identifies the message as a password response. Note that
                 this is also used for GSSAPI, SSPI and SASL response messages.
                 The exact message type can be deduced from the context.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The password (encrypted, if requested).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 PortalSuspended (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('s')
 </term>
 <listitem>
 <para>
                 Identifies the message as a portal-suspended indicator.
                 Note this only appears if an Execute message's row-count limit
                 was reached.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Query (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('Q')
 </term>
 <listitem>
 <para>
                 Identifies the message as a simple query.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The query string itself.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 ReadyForQuery (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('Z')
 </term>
 <listitem>
 <para>
                 Identifies the message type.  ReadyForQuery is sent
                 whenever the backend is ready for a new query cycle.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(5)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte1
 </term>
 <listitem>
 <para>
                 Current backend transaction status indicator.
                 Possible values are '<literal>I</literal>' if idle (not in
                 a transaction block); '<literal>T</literal>' if in a transaction
                 block; or '<literal>E</literal>' if in a failed transaction
                 block (queries will be rejected until block is ended).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 RowDescription (B)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('T')
 </term>
 <listitem>
 <para>
                 Identifies the message as a row description.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 Specifies the number of fields in a row (can be zero).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Then, for each field, there is the following:
 <variablelist>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The field name.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 If the field can be identified as a column of a specific
                 table, the object ID of the table; otherwise zero.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 If the field can be identified as a column of a specific
                 table, the attribute number of the column; otherwise zero.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The object ID of the field's data type.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The data type size (see <varname>pg_type.typlen</varname>).
                 Note that negative values denote variable-width types.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 The type modifier (see <varname>pg_attribute.atttypmod</varname>).
                 The meaning of the modifier is type-specific.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 The format code being used for the field.  Currently will
                 be zero (text) or one (binary).  In a RowDescription
                 returned from the statement variant of Describe, the
                 format code is not yet known and will always be zero.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 SASLInitialResponse (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('p')
 </term>
 <listitem>
 <para>
                 Identifies the message as an initial SASL response. Note that
                 this is also used for GSSAPI, SSPI and password response messages.
                 The exact message type is deduced from the context.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Name of the SASL authentication mechanism that the client
                 selected.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of SASL mechanism specific "Initial Client Response" that
                 follows, or -1 if there is no Initial Response.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 SASL mechanism specific "Initial Response".
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 SASLResponse (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('p')
 </term>
 <listitem>
 <para>
                 Identifies the message as a SASL response. Note that
                 this is also used for GSSAPI, SSPI and password response messages.
                 The exact message type can be deduced from the context.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 SASL mechanism specific message data.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 SSLRequest (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Int32(8)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(80877103)
 </term>
 <listitem>
 <para>
                 The <acronym>SSL</acronym> request code.  The value is chosen to contain
                 <literal>1234</literal> in the most significant 16 bits, and <literal>5679</literal> in the
                 least significant 16 bits.  (To avoid confusion, this code
                 must not be the same as any protocol version number.)
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 StartupMessage (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(196608)
 </term>
 <listitem>
 <para>
                 The protocol version number.  The most significant 16 bits are
                 the major version number (3 for the protocol described here).
                 The least significant 16 bits are the minor version number
                 (0 for the protocol described here).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         The protocol version number is followed by one or more pairs of
         parameter name and value strings.  A zero byte is required as a
         terminator after the last name/value pair.
         Parameters can appear in any
         order.  <literal>user</literal> is required, others are optional.
         Each parameter is specified as:
 <variablelist>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The parameter name.  Currently recognized names are:

 <variablelist>
 <varlistentry>
 <term>
                 <literal>user</literal>
 </term>
 <listitem>
 <para>
                         The database user name to connect as.  Required;
                         there is no default.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
                 <literal>database</literal>
 </term>
 <listitem>
 <para>
                         The database to connect to.  Defaults to the user name.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
                 <literal>options</literal>
 </term>
 <listitem>
 <para>
                         Command-line arguments for the backend.  (This is
                         deprecated in favor of setting individual run-time
                         parameters.)  Spaces within this string are
                         considered to separate arguments, unless escaped with
                         a backslash (<literal>\</literal>); write <literal>\\</literal> to
                         represent a literal backslash.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
                 <literal>replication</literal>
 </term>
 <listitem>
 <para>
                         Used to connect in streaming replication mode, where
                         a small set of replication commands can be issued
                         instead of SQL statements. Value can be
                         <literal>true</literal>, <literal>false</literal>, or
                         <literal>database</literal>, and the default is
                         <literal>false</literal>. See
                         <xref linkend="protocol-replication"/> for details.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

                 In addition to the above, other parameters may be listed.
                 Parameter names beginning with <literal>_pq_.</literal> are
                 reserved for use as protocol extensions, while others are
                 treated as run-time parameters to be set at backend start
                 time.  Such settings will be applied during backend start
                 (after parsing the command-line arguments if any) and will
                 act as session defaults.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The parameter value.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Sync (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('S')
 </term>
 <listitem>
 <para>
                 Identifies the message as a Sync command.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 <varlistentry>
 <term>
 Terminate (F)
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('X')
 </term>
 <listitem>
 <para>
                 Identifies the message as a termination.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32(4)
 </term>
 <listitem>
 <para>
                 Length of message contents in bytes, including self.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>

 </para>
 </listitem>
 </varlistentry>


 </variablelist>

 </sect1>


 <sect1 id="protocol-error-fields">
 <title>Error and Notice Message Fields</title>

 <para>
 This section describes the fields that can appear in ErrorResponse and
 NoticeResponse messages.  Each field type has a single-byte identification
 token.  Note that any given field type should appear at most once per
 message.
 </para>

 <variablelist>

 <varlistentry>
 <term>
 <literal>S</literal>
 </term>
 <listitem>
 <para>
         Severity: the field contents are
         <literal>ERROR</literal>, <literal>FATAL</literal>, or
         <literal>PANIC</literal> (in an error message), or
         <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
         <literal>INFO</literal>, or <literal>LOG</literal> (in a notice message),
         or a localized translation of one of these.  Always present.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>V</literal>
 </term>
 <listitem>
 <para>
         Severity: the field contents are
         <literal>ERROR</literal>, <literal>FATAL</literal>, or
         <literal>PANIC</literal> (in an error message), or
         <literal>WARNING</literal>, <literal>NOTICE</literal>, <literal>DEBUG</literal>,
         <literal>INFO</literal>, or <literal>LOG</literal> (in a notice message).
         This is identical to the <literal>S</literal> field except
         that the contents are never localized.  This is present only in
         messages generated by <productname>PostgreSQL</productname> versions 9.6
         and later.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>C</literal>
 </term>
 <listitem>
 <para>
         Code: the SQLSTATE code for the error (see <xref
         linkend="errcodes-appendix"/>).  Not localizable.  Always present.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>M</literal>
 </term>
 <listitem>
 <para>
         Message: the primary human-readable error message.
         This should be accurate but terse (typically one line).
         Always present.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>D</literal>
 </term>
 <listitem>
 <para>
         Detail: an optional secondary error message carrying more
         detail about the problem.  Might run to multiple lines.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>H</literal>
 </term>
 <listitem>
 <para>
         Hint: an optional suggestion what to do about the problem.
         This is intended to differ from Detail in that it offers advice
         (potentially inappropriate) rather than hard facts.
         Might run to multiple lines.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>P</literal>
 </term>
 <listitem>
 <para>
         Position: the field value is a decimal ASCII integer, indicating
         an error cursor position as an index into the original query string.
         The first character has index 1, and positions are measured in
         characters not bytes.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>p</literal>
 </term>
 <listitem>
 <para>
         Internal position: this is defined the same as the <literal>P</literal>
         field, but it is used when the cursor position refers to an internally
         generated command rather than the one submitted by the client.
         The <literal>q</literal> field will always appear when this field appears.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>q</literal>
 </term>
 <listitem>
 <para>
         Internal query: the text of a failed internally-generated command.
         This could be, for example, an SQL query issued by a PL/pgSQL function.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>W</literal>
 </term>
 <listitem>
 <para>
         Where: an indication of the context in which the error occurred.
         Presently this includes a call stack traceback of active
         procedural language functions and internally-generated queries.
         The trace is one entry per line, most recent first.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>s</literal>
 </term>
 <listitem>
 <para>
         Schema name: if the error was associated with a specific database
         object, the name of the schema containing that object, if any.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>t</literal>
 </term>
 <listitem>
 <para>
         Table name: if the error was associated with a specific table, the
         name of the table.  (Refer to the schema name field for the name of
         the table's schema.)
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>c</literal>
 </term>
 <listitem>
 <para>
         Column name: if the error was associated with a specific table column,
         the name of the column.  (Refer to the schema and table name fields to
         identify the table.)
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>d</literal>
 </term>
 <listitem>
 <para>
         Data type name: if the error was associated with a specific data type,
         the name of the data type.  (Refer to the schema name field for the
         name of the data type's schema.)
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>n</literal>
 </term>
 <listitem>
 <para>
         Constraint name: if the error was associated with a specific
         constraint, the name of the constraint.  Refer to fields listed above
         for the associated table or domain.  (For this purpose, indexes are
         treated as constraints, even if they weren't created with constraint
         syntax.)
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>F</literal>
 </term>
 <listitem>
 <para>
         File: the file name of the source-code location where the error
         was reported.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>L</literal>
 </term>
 <listitem>
 <para>
         Line: the line number of the source-code location where the error
         was reported.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 <literal>R</literal>
 </term>
 <listitem>
 <para>
         Routine: the name of the source-code routine reporting the error.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>

 <note>
  <para>
   The fields for schema name, table name, column name, data type name, and
   constraint name are supplied only for a limited number of error types;
   see <xref linkend="errcodes-appendix"/>.  Frontends should not assume that
   the presence of any of these fields guarantees the presence of another
   field.  Core error sources observe the interrelationships noted above, but
   user-defined functions may use these fields in other ways.  In the same
   vein, clients should not assume that these fields denote contemporary
   objects in the current database.
  </para>
 </note>

 <para>
 The client is responsible for formatting displayed information to meet its
 needs; in particular it should break long lines as needed.  Newline characters
 appearing in the error message fields should be treated as paragraph breaks,
 not line breaks.
 </para>

 </sect1>

 <sect1 id="protocol-logicalrep-message-formats">
 <title>Logical Replication Message Formats</title>

 <para>
 This section describes the detailed format of each logical replication message.
 These messages are returned either by the replication slot SQL interface or are
 sent by a walsender. In case of a walsender they are encapsulated inside the replication
 protocol WAL messages as described in <xref linkend="protocol-replication"/>
 and generally obey same message flow as physical replication.
 </para>

 <variablelist>

 <varlistentry>
 <term>
 Begin
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('B')
 </term>
 <listitem>
 <para>
                 Identifies the message as a begin message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 The final LSN of the transaction.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 Commit timestamp of the transaction. The value is in number
                 of microseconds since PostgreSQL epoch (2000-01-01).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Xid of the transaction.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Message
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('M')
 </term>
 <listitem>
 <para>
                 Identifies the message as a logical decoding message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Xid of the transaction (only present for streamed transactions).
                 This field is available since protocol version 2.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 Flags; Either 0 for no flags or 1 if the logical decoding
                 message is transactional.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 The LSN of the logical decoding message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 The prefix of the logical decoding message.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of the content.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 The content of the logical decoding message.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Commit
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('C')
 </term>
 <listitem>
 <para>
                 Identifies the message as a commit message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 Flags; currently unused (must be 0).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 The LSN of the commit.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 The end LSN of the transaction.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 Commit timestamp of the transaction. The value is in number
                 of microseconds since PostgreSQL epoch (2000-01-01).
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Origin
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('O')
 </term>
 <listitem>
 <para>
                 Identifies the message as an origin message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 The LSN of the commit on the origin server.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Name of the origin.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>

 <para>
   Note that there can be multiple Origin messages inside a single transaction.
 </para>

 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Relation
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('R')
 </term>
 <listitem>
 <para>
                 Identifies the message as a relation message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                Xid of the transaction (only present for streamed transactions).
                This field is available since protocol version 2.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 ID of the relation.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Namespace (empty string for <literal>pg_catalog</literal>).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Relation name.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 Replica identity setting for the relation (same as
                 <structfield>relreplident</structfield> in <structname>pg_class</structname>).
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 Number of columns.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Next, the following message part appears for each column (except generated columns):
 <variablelist>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 Flags for the column. Currently can be either 0 for no flags
                 or 1 which marks the column as part of the key.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Name of the column.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 ID of the column's data type.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Type modifier of the column (<structfield>atttypmod</structfield>).
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Type
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('Y')
 </term>
 <listitem>
 <para>
                 Identifies the message as a type message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                Xid of the transaction (only present for streamed transactions).
                This field is available since protocol version 2.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 ID of the data type.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Namespace (empty string for <literal>pg_catalog</literal>).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         String
 </term>
 <listitem>
 <para>
                 Name of the data type.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Insert
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('I')
 </term>
 <listitem>
 <para>
                 Identifies the message as an insert message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Xid of the transaction (only present for streamed transactions).
                 This field is available since protocol version 2.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 ID of the relation corresponding to the ID in the relation
                 message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte1('N')
 </term>
 <listitem>
 <para>
                 Identifies the following TupleData message as a new tuple.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         TupleData
 </term>
 <listitem>
 <para>
                 TupleData message part representing the contents of new tuple.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Update
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('U')
 </term>
 <listitem>
 <para>
                 Identifies the message as an update message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                Xid of the transaction (only present for streamed transactions).
                This field is available since protocol version 2.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 ID of the relation corresponding to the ID in the relation
                 message.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Byte1('K')
 </term>
 <listitem>
 <para>
                 Identifies the following TupleData submessage as a key.
                 This field is optional and is only present if
                 the update changed data in any of the column(s) that are
                 part of the REPLICA IDENTITY index.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Byte1('O')
 </term>
 <listitem>
 <para>
                 Identifies the following TupleData submessage as an old tuple.
                 This field is optional and is only present if table in which
                 the update happened has REPLICA IDENTITY set to FULL.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         TupleData
 </term>
 <listitem>
 <para>
                 TupleData message part representing the contents of the old tuple
                 or primary key. Only present if the previous 'O' or 'K' part
                 is present.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Byte1('N')
 </term>
 <listitem>
 <para>
                 Identifies the following TupleData message as a new tuple.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         TupleData
 </term>
 <listitem>
 <para>
                 TupleData message part representing the contents of a new tuple.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>

 <para>
     The Update message may contain either a 'K' message part or an 'O' message part
     or neither of them, but never both of them.
 </para>

 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Delete
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('D')
 </term>
 <listitem>
 <para>
                 Identifies the message as a delete message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                Xid of the transaction (only present for streamed transactions).
                This field is available since protocol version 2.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 ID of the relation corresponding to the ID in the relation
                 message.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Byte1('K')
 </term>
 <listitem>
 <para>
                 Identifies the following TupleData submessage as a key.
                 This field is present if the table in which the delete has
                 happened uses an index as REPLICA IDENTITY.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         Byte1('O')
 </term>
 <listitem>
 <para>
                 Identifies the following TupleData message as an old tuple.
                 This field is present if the table in which the delete
                 happened has REPLICA IDENTITY set to FULL.
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
         TupleData
 </term>
 <listitem>
 <para>
                 TupleData message part representing the contents of the old tuple
                 or primary key, depending on the previous field.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
 </para>

 <para>
     The Delete message may contain either a 'K' message part or an 'O' message part,
     but never both of them.
 </para>

 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Truncate
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('T')
 </term>
 <listitem>
 <para>
                 Identifies the message as a truncate message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                Xid of the transaction (only present for streamed transactions).
                This field is available since protocol version 2.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Number of relations
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 Option bits for <command>TRUNCATE</command>:
                 1 for <literal>CASCADE</literal>, 2 for <literal>RESTART IDENTITY</literal>
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 ID of the relation corresponding to the ID in the relation
                 message.  This field is repeated for each relation.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 </variablelist>

 <para>

 The following messages (Stream Start, Stream Stop, Stream Commit, and
 Stream Abort) are available since protocol version 2.

 </para>

 <variablelist>

 <varlistentry>
 <term>
 Stream Start
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('S')
 </term>
 <listitem>
 <para>
                 Identifies the message as a stream start message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Xid of the transaction.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 A value of 1 indicates this is the first stream segment for
                 this XID, 0 for any other stream segment.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Stream Stop
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('E')
 </term>
 <listitem>
 <para>
                 Identifies the message as a stream stop message.
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Stream Commit
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('c')
 </term>
 <listitem>
 <para>
                 Identifies the message as a stream commit message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Xid of the transaction.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int8
 </term>
 <listitem>
 <para>
                 Flags; currently unused (must be 0).
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 The LSN of the commit.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 The end LSN of the transaction.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int64
 </term>
 <listitem>
 <para>
                 Commit timestamp of the transaction. The value is in number
                 of microseconds since PostgreSQL epoch (2000-01-01).
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 <varlistentry>
 <term>
 Stream Abort
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Byte1('A')
 </term>
 <listitem>
 <para>
                 Identifies the message as a stream abort message.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Xid of the transaction.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Xid of the subtransaction (will be same as xid of the transaction for top-level
                 transactions).
 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 </variablelist>

 <para>

 The following message parts are shared by the above messages.

 </para>

 <variablelist>

 <varlistentry>
 <term>
 TupleData
 </term>
 <listitem>
 <para>

 <variablelist>
 <varlistentry>
 <term>
         Int16
 </term>
 <listitem>
 <para>
                 Number of columns.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Next, one of the following submessages appears for each column (except generated columns):
 <variablelist>
 <varlistentry>
 <term>
         Byte1('n')
 </term>
 <listitem>
 <para>
                 Identifies the data as NULL value.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Or
 <variablelist>
 <varlistentry>
 <term>
         Byte1('u')
 </term>
 <listitem>
 <para>
                 Identifies unchanged TOASTed value (the actual value is not
                 sent).
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Or
 <variablelist>
 <varlistentry>
 <term>
         Byte1('t')
 </term>
 <listitem>
 <para>
                 Identifies the data as text formatted value.
 </para>
 </listitem>
 </varlistentry>
 </variablelist>
         Or
 <variablelist>
 <varlistentry>
 <term>
         Byte1('b')
 </term>
 <listitem>
 <para>
                 Identifies the data as binary formatted value.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Int32
 </term>
 <listitem>
 <para>
                 Length of the column value.
 </para>
 </listitem>
 </varlistentry>
 <varlistentry>
 <term>
         Byte<replaceable>n</replaceable>
 </term>
 <listitem>
 <para>
                 The value of the column, either in binary or in text format.
                 (As specified in the preceding format byte).
                 <replaceable>n</replaceable> is the above length.

 </para>
 </listitem>
 </varlistentry>

 </variablelist>
 </para>
 </listitem>
 </varlistentry>

 </variablelist>

 </sect1>

 <sect1 id="protocol-changes">
 <title>Summary of Changes since Protocol 2.0</title>

 <para>
 This section provides a quick checklist of changes, for the benefit of
 developers trying to update existing client libraries to protocol 3.0.
 </para>

 <para>
 The initial startup packet uses a flexible list-of-strings format
 instead of a fixed format.  Notice that session default values for run-time
 parameters can now be specified directly in the startup packet.  (Actually,
 you could do that before using the <literal>options</literal> field, but given the
 limited width of <literal>options</literal> and the lack of any way to quote
 whitespace in the values, it wasn't a very safe technique.)
 </para>

 <para>
 All messages now have a length count immediately following the message type
 byte (except for startup packets, which have no type byte).  Also note that
 PasswordMessage now has a type byte.
 </para>

 <para>
 ErrorResponse and NoticeResponse ('<literal>E</literal>' and '<literal>N</literal>')
 messages now contain multiple fields, from which the client code can
 assemble an error message of the desired level of verbosity.  Note that
 individual fields will typically not end with a newline, whereas the single
 string sent in the older protocol always did.
 </para>

 <para>
 The ReadyForQuery ('<literal>Z</literal>') message includes a transaction status
 indicator.
 </para>

 <para>
 The distinction between BinaryRow and DataRow message types is gone; the
 single DataRow message type serves for returning data in all formats.
 Note that the layout of DataRow has changed to make it easier to parse.
 Also, the representation of binary values has changed: it is no longer
 directly tied to the server's internal representation.
 </para>

 <para>
 There is a new <quote>extended query</quote> sub-protocol, which adds the frontend
 message types Parse, Bind, Execute, Describe, Close, Flush, and Sync, and the
 backend message types ParseComplete, BindComplete, PortalSuspended,
 ParameterDescription, NoData, and CloseComplete.  Existing clients do not
 have to concern themselves with this sub-protocol, but making use of it
 might allow improvements in performance or functionality.
 </para>

 <para>
 <command>COPY</command> data is now encapsulated into CopyData and CopyDone messages.  There
 is a well-defined way to recover from errors during <command>COPY</command>.  The special
 <quote><literal>\.</literal></quote> last line is not needed anymore, and is not sent
 during <command>COPY OUT</command>.
 (It is still recognized as a terminator during <command>COPY IN</command>, but its use is
 deprecated and will eventually be removed.)  Binary <command>COPY</command> is supported.
 The CopyInResponse and CopyOutResponse messages include fields indicating
 the number of columns and the format of each column.
 </para>

 <para>
 The layout of FunctionCall and FunctionCallResponse messages has changed.
 FunctionCall can now support passing NULL arguments to functions.  It also
 can handle passing parameters and retrieving results in either text or
 binary format.  There is no longer any reason to consider FunctionCall a
 potential security hole, since it does not offer direct access to internal
 server data representations.
 </para>

 <para>
 The backend sends ParameterStatus ('<literal>S</literal>') messages during connection
 startup for all parameters it considers interesting to the client library.
 Subsequently, a ParameterStatus message is sent whenever the active value
 changes for any of these parameters.
 </para>

 <para>
 The RowDescription ('<literal>T</literal>') message carries new table OID and column
 number fields for each column of the described row.  It also shows the format
 code for each column.
 </para>

 <para>
 The CursorResponse ('<literal>P</literal>') message is no longer generated by
 the backend.
 </para>

 <para>
 The NotificationResponse ('<literal>A</literal>') message has an additional string
 field, which can carry a <quote>payload</quote> string passed
 from the <command>NOTIFY</command> event sender.
 </para>

 <para>
 The EmptyQueryResponse ('<literal>I</literal>') message used to include an empty
 string parameter; this has been removed.
 </para>

 </sect1>

 </chapter>