doc/src/sgml/ref/declare.sgml

<!--
doc/src/sgml/ref/declare.sgml
PostgreSQL documentation
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<refentry id="sql-declare">
 <indexterm zone="sql-declare">
  <primary>DECLARE</primary>
 </indexterm>

 <indexterm zone="sql-declare">
  <primary>cursor</primary>
  <secondary>DECLARE</secondary>
 </indexterm>

 <refmeta>
  <refentrytitle>DECLARE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>DECLARE</refname>
  <refpurpose>define a cursor or a parallel retrieve cursor</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
DECLARE <replaceable class="parameter">name</replaceable> [ BINARY ] [ ASENSITIVE | INSENSITIVE ] [ [ NO ] SCROLL ]
  [ PARALLEL RETRIEVE ] CURSOR [ { WITH | WITHOUT } HOLD ] FOR <replaceable class="parameter">query</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>DECLARE</command> allows a user to create cursors, which
   can be used to retrieve
   a small number of rows at a time out of a larger query.
   After the cursor is created, rows are fetched from it using
   <link linkend="sql-fetch"><command>FETCH</command></link>.
  </para>

  <para>
    Like a normal cursor, user can declare a parallel retrieve cursor on
    coordinator, then retrieve the query results on each segment directly.
  </para>

  <para>
    Parallel retrieve cursor has similar declaration and syntax as normal cursor
    does. However, some cursor operations are not supported in parallel retrieve
    cursor(e.g. MOVE).
  </para>

  <para>
    NOTE: Orca doesn't support PARALLEL RETRIEVE CURSOR for now. It would fall
    back to postgres optimizer automatically.
  </para>

  <note>
   <para>
    This page describes usage of cursors at the SQL command level.
    If you are trying to use cursors inside a <application>PL/pgSQL</application>
    function, the rules are different &mdash;
    see <xref linkend="plpgsql-cursors"/>.
   </para>
  </note>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      The name of the cursor to be created.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>BINARY</literal></term>
    <listitem>
     <para>
      Causes the cursor to return data in binary rather than in text format.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>ASENSITIVE</literal></term>
    <term><literal>INSENSITIVE</literal></term>
    <listitem>
     <para>
      Cursor sensitivity determines whether changes to the data underlying the
      cursor, done in the same transaction, after the cursor has been
      declared, are visible in the cursor.  <literal>INSENSITIVE</literal>
      means they are not visible, <literal>ASENSITIVE</literal> means the
      behavior is implementation-dependent.  A third behavior,
      <literal>SENSITIVE</literal>, meaning that such changes are visible in
      the cursor, is not available in <productname>PostgreSQL</productname>.
      In <productname>PostgreSQL</productname>, all cursors are insensitive;
      so these key words have no effect and are only accepted for
      compatibility with the SQL standard.
     </para>

     <para>
      Specifying <literal>INSENSITIVE</literal> together with <literal>FOR
      UPDATE</literal> or <literal>FOR SHARE</literal> is an error.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SCROLL</literal></term>
    <term><literal>NO SCROLL</literal></term>
    <listitem>
     <para><literal>SCROLL</literal> specifies that the cursor can be used
      to retrieve rows in a nonsequential fashion (e.g.,
      backward). Depending upon the complexity of the query's
      execution plan, specifying <literal>SCROLL</literal> might impose
      a performance penalty on the query's execution time.
      <literal>NO SCROLL</literal> specifies that the cursor cannot be
      used to retrieve rows in a nonsequential fashion.  The default is to
      allow scrolling in some cases; this is not the same as specifying
      <literal>SCROLL</literal>. See <xref linkend="sql-declare-notes"/>
      below for details.
     </para>

     <para>
      PARALLEL RETRIEVE CURSOR with <literal>WITH SCROLL</literal> is not supported.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>WITH HOLD</literal></term>
    <term><literal>WITHOUT HOLD</literal></term>
    <listitem>
     <para><literal>WITH HOLD</literal> specifies that the cursor can
      continue to be used after the transaction that created it
      successfully commits.  <literal>WITHOUT HOLD</literal> specifies
      that the cursor cannot be used outside of the transaction that
      created it. If neither <literal>WITHOUT HOLD</literal> nor
      <literal>WITH HOLD</literal> is specified, <literal>WITHOUT
      HOLD</literal> is the default.
     </para>

     <para>
      PARALLEL RETRIEVE CURSOR with <literal>WITH HOLD</literal> is not supported.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">query</replaceable></term>
    <listitem>
     <para>
      A <link linkend="sql-select"><command>SELECT</command></link> or
      <link linkend="sql-values"><command>VALUES</command></link> command
      which will provide the rows to be returned by the cursor.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The key words <literal>ASENSITIVE</literal>, <literal>BINARY</literal>,
   <literal>INSENSITIVE</literal>, and <literal>SCROLL</literal> can
   appear in any order.
  </para>
 </refsect1>

 <refsect1 id="sql-declare-notes" xreflabel="Notes">
  <title>Notes</title>

  <para>
   Normal cursors return data in text format, the same as a
   <command>SELECT</command> would produce.  The <literal>BINARY</literal> option
   specifies that the cursor should return data in binary format.
   This reduces conversion effort for both the server and client,
   at the cost of more programmer effort to deal with platform-dependent
   binary data formats.
   As an example, if a query returns a value of one from an integer column,
   you would get a string of <literal>1</literal> with a default cursor,
   whereas with a binary cursor you would get
   a 4-byte field containing the internal representation of the value
   (in big-endian byte order).
  </para>

  <para>
   Binary cursors should be used carefully.  Many applications,
   including <application>psql</application>, are not prepared to
   handle binary cursors and expect data to come back in the text
   format.
  </para>

  <note>
   <para>
    When the client application uses the <quote>extended query</quote> protocol
    to issue a <command>FETCH</command> command, the Bind protocol message
    specifies whether data is to be retrieved in text or binary format.
    This choice overrides the way that the cursor is defined.  The concept
    of a binary cursor as such is thus obsolete when using extended query
    protocol &mdash; any cursor can be treated as either text or binary.
   </para>
  </note>

   <para>
    Unless <literal>WITH HOLD</literal> is specified, the cursor
    created by this command can only be used within the current
    transaction.  Thus, <command>DECLARE</command> without <literal>WITH
    HOLD</literal> is useless outside a transaction block: the cursor would
    survive only to the completion of the statement.  Therefore
    <productname>PostgreSQL</productname> reports an error if such a
    command is used outside a transaction block.
    Use
    <link linkend="sql-begin"><command>BEGIN</command></link> and
    <link linkend="sql-commit"><command>COMMIT</command></link>
    (or <link linkend="sql-rollback"><command>ROLLBACK</command></link>)
    to define a transaction block.
   </para>

   <para>
    If <literal>WITH HOLD</literal> is specified and the transaction
    that created the cursor successfully commits, the cursor can
    continue to be accessed by subsequent transactions in the same
    session.  (But if the creating transaction is aborted, the cursor
    is removed.)  A cursor created with <literal>WITH HOLD</literal>
    is closed when an explicit <command>CLOSE</command> command is
    issued on it, or the session ends.  In the current implementation,
    the rows represented by a held cursor are copied into a temporary
    file or memory area so that they remain available for subsequent
    transactions.
   </para>

   <para>
    <literal>WITH HOLD</literal> may not be specified when the query
    includes <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal>.
   </para>

   <para>
    The <literal>SCROLL</literal> option should be specified when defining a
    cursor that will be used to fetch backwards.  This is required by
    the SQL standard.  However, for compatibility with earlier
    versions, <productname>PostgreSQL</productname> will allow
    backward fetches without <literal>SCROLL</literal>, if the cursor's query
    plan is simple enough that no extra overhead is needed to support
    it. However, application developers are advised not to rely on
    using backward fetches from a cursor that has not been created
    with <literal>SCROLL</literal>.  If <literal>NO SCROLL</literal> is
    specified, then backward fetches are disallowed in any case.
   </para>

   <para>
    Backward fetches are also disallowed when the query
    includes <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal>; therefore
    <literal>SCROLL</literal> may not be specified in this case.
   </para>

   <caution>
    <para>
     Scrollable cursors may give unexpected
     results if they invoke any volatile functions (see <xref
     linkend="xfunc-volatility"/>).  When a previously fetched row is
     re-fetched, the functions might be re-executed, perhaps leading to
     results different from the first time.  It's best to
     specify <literal>NO SCROLL</literal> for a query involving volatile
     functions.  If that is not practical, one workaround
     is to declare the cursor <literal>SCROLL WITH HOLD</literal> and commit the
     transaction before reading any rows from it.  This will force the
     entire output of the cursor to be materialized in temporary storage,
     so that volatile functions are executed exactly once for each row.
    </para>
   </caution>

   <para>
    If the cursor's query includes <literal>FOR UPDATE</literal> or <literal>FOR
    SHARE</literal>, then returned rows are locked at the time they are first
    fetched, in the same way as for a regular
    <link linkend="sql-select"><command>SELECT</command></link> command with
    these options.
    In addition, the returned rows will be the most up-to-date versions.
   </para>

   <caution>
    <para>
     It is generally recommended to use <literal>FOR UPDATE</literal> if the cursor
     is intended to be used with <command>UPDATE ... WHERE CURRENT OF</command> or
     <command>DELETE ... WHERE CURRENT OF</command>.  Using <literal>FOR UPDATE</literal>
     prevents other sessions from changing the rows between the time they are
     fetched and the time they are updated.  Without <literal>FOR UPDATE</literal>,
     a subsequent <literal>WHERE CURRENT OF</literal> command will have no effect if
     the row was changed since the cursor was created.
    </para>

    <para>
     Another reason to use <literal>FOR UPDATE</literal> is that without it, a
     subsequent <literal>WHERE CURRENT OF</literal> might fail if the cursor query
     does not meet the SQL standard's rules for being <quote>simply
     updatable</quote> (in particular, the cursor must reference just one table
     and not use grouping or <literal>ORDER BY</literal>).  Cursors
     that are not simply updatable might work, or might not, depending on plan
     choice details; so in the worst case, an application might work in testing
     and then fail in production.  If <literal>FOR UPDATE</literal> is
     specified, the cursor is guaranteed to be updatable.
    </para>

    <para>
     The main reason not to use <literal>FOR UPDATE</literal> with <literal>WHERE
     CURRENT OF</literal> is if you need the cursor to be scrollable, or to be
     isolated from concurrent updates (that is, continue to show the old
     data).  If this is a requirement, pay close heed to the caveats shown
     above.
    </para>
   </caution>

   <para>
    The SQL standard only makes provisions for cursors in embedded
    <acronym>SQL</acronym>.  The <productname>PostgreSQL</productname>
    server does not implement an <command>OPEN</command> statement for
    cursors; a cursor is considered to be open when it is declared.
    However, <application>ECPG</application>, the embedded SQL
    preprocessor for <productname>PostgreSQL</productname>, supports
    the standard SQL cursor conventions, including those involving
    <command>DECLARE</command> and <command>OPEN</command> statements.
   </para>

   <para>
    You can see all available cursors by querying the <link
    linkend="view-pg-cursors"><structname>pg_cursors</structname></link>
    system view.
   </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To declare a cursor:
<programlisting>
DECLARE liahona CURSOR FOR SELECT * FROM films;
</programlisting>
   See <xref linkend="sql-fetch"/> for more
   examples of cursor usage.
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The SQL standard allows cursors only in embedded
   <acronym>SQL</acronym> and in modules. <productname>PostgreSQL</productname>
   permits cursors to be used interactively.
  </para>

  <para>
   According to the SQL standard, changes made to insensitive cursors by
   <literal>UPDATE ... WHERE CURRENT OF</literal> and <literal>DELETE
   ... WHERE CURRENT OF</literal> statements are visible in that same
   cursor.  <productname>PostgreSQL</productname> treats these statements like
   all other data changing statements in that they are not visible in
   insensitive cursors.
  </para>

  <para>
   Binary cursors are a <productname>PostgreSQL</productname>
   extension.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-close"/></member>
   <member><xref linkend="sql-fetch"/></member>
   <member><xref linkend="sql-move"/></member>
  </simplelist>
 </refsect1>
</refentry>