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 <!--
 doc/src/sgml/ref/lock.sgml
 PostgreSQL documentation
 -->

 <refentry id="sql-lock">
  <indexterm zone="sql-lock">
   <primary>LOCK</primary>
  </indexterm>

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

  <refnamediv>
   <refname>LOCK</refname>
   <refpurpose>lock a table</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
 <synopsis>
 LOCK [ TABLE ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ] [, ...] [ IN <replaceable class="parameter">lockmode</replaceable> MODE ] [ NOWAIT ]

 <phrase>where <replaceable class="parameter">lockmode</replaceable> is one of:</phrase>

     ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
     | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
 </synopsis>
  </refsynopsisdiv>

  <refsect1>
   <title>Description</title>

   <para>
    <command>LOCK TABLE</command> obtains a table-level lock, waiting
    if necessary for any conflicting locks to be released.  If
    <literal>NOWAIT</literal> is specified, <command>LOCK
    TABLE</command> does not wait to acquire the desired lock: if it
    cannot be acquired immediately, the command is aborted and an
    error is emitted.  Once obtained, the lock is held for the
    remainder of the current transaction.  (There is no <command>UNLOCK
    TABLE</command> command; locks are always released at transaction
    end.)
   </para>

   <para>
    When a view is locked, all relations appearing in the view definition
    query are also locked recursively with the same lock mode.
   </para>

   <para>
    When acquiring locks automatically for commands that reference
    tables, <productname>PostgreSQL</productname> always uses the least
    restrictive lock mode possible. <command>LOCK TABLE</command>
    provides for cases when you might need more restrictive locking.
    For example, suppose an application runs a transaction at the
    <literal>READ COMMITTED</literal> isolation level and needs to ensure that
    data in a table remains stable for the duration of the transaction.
    To achieve this you could obtain <literal>SHARE</literal> lock mode over the
    table before querying. This will prevent concurrent data changes
    and ensure subsequent reads of the table see a stable view of
    committed data, because <literal>SHARE</literal> lock mode conflicts with
    the <literal>ROW EXCLUSIVE</literal> lock acquired by writers, and your
    <command>LOCK TABLE <replaceable
    class="parameter">name</replaceable> IN SHARE MODE</command>
    statement will wait until any concurrent holders of <literal>ROW
    EXCLUSIVE</literal> mode locks commit or roll back. Thus, once you
    obtain the lock, there are no uncommitted writes outstanding;
    furthermore none can begin until you release the lock.
   </para>

   <para>
    To achieve a similar effect when running a transaction at the
    <literal>REPEATABLE READ</literal> or <literal>SERIALIZABLE</literal>
    isolation level, you have to execute the <command>LOCK TABLE</command> statement
    before executing any <command>SELECT</command> or data modification statement.
    A <literal>REPEATABLE READ</literal> or <literal>SERIALIZABLE</literal> transaction's
    view of data will be frozen when its first
    <command>SELECT</command> or data modification statement begins.  A <command>LOCK
    TABLE</command> later in the transaction will still prevent concurrent writes
    &mdash; but it won't ensure that what the transaction reads corresponds to
    the latest committed values.
   </para>

   <para>
    If a transaction of this sort is going to change the data in the
    table, then it should use <literal>SHARE ROW EXCLUSIVE</literal> lock mode
    instead of <literal>SHARE</literal> mode.  This ensures that only one
    transaction of this type runs at a time.  Without this, a deadlock
    is possible: two transactions might both acquire <literal>SHARE</literal>
    mode, and then be unable to also acquire <literal>ROW EXCLUSIVE</literal>
    mode to actually perform their updates.  (Note that a transaction's
    own locks never conflict, so a transaction can acquire <literal>ROW
    EXCLUSIVE</literal> mode when it holds <literal>SHARE</literal> mode &mdash; but not
    if anyone else holds <literal>SHARE</literal> mode.)  To avoid deadlocks,
    make sure all transactions acquire locks on the same objects in the
    same order, and if multiple lock modes are involved for a single
    object, then transactions should always acquire the most
    restrictive mode first.
   </para>

   <para>
    More information about the lock modes and locking strategies can be
    found in <xref linkend="explicit-locking"/>.
   </para>
  </refsect1>

  <refsect1>
   <title>Parameters</title>

   <variablelist>
    <varlistentry>
     <term><replaceable class="parameter">name</replaceable></term>
     <listitem>
      <para>
       The name (optionally schema-qualified) of an existing table to
       lock. If <literal>ONLY</literal> is specified before the table name, only that
       table is locked. If <literal>ONLY</literal> is not specified, the table and all
       its descendant tables (if any) are locked.  Optionally, <literal>*</literal>
       can be specified after the table name to explicitly indicate that
       descendant tables are included.
      </para>

      <para>
       The command <literal>LOCK TABLE a, b;</literal> is equivalent to
       <literal>LOCK TABLE a; LOCK TABLE b;</literal>. The tables are locked
       one-by-one in the order specified in the <command>LOCK
       TABLE</command> command.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">lockmode</replaceable></term>
     <listitem>
      <para>
       The lock mode specifies which locks this lock conflicts with.
       Lock modes are described in <xref linkend="explicit-locking"/>.
      </para>

      <para>
       If no lock mode is specified, then <literal>ACCESS
       EXCLUSIVE</literal>, the most restrictive mode, is used.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>NOWAIT</literal></term>
     <listitem>
      <para>
       Specifies that <command>LOCK TABLE</command> should not wait for
       any conflicting locks to be released: if the specified lock(s)
       cannot be acquired immediately without waiting, the transaction
       is aborted.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </refsect1>

  <refsect1>
   <title>Notes</title>

    <para>
     <literal>LOCK TABLE ... IN ACCESS SHARE MODE</literal> requires <literal>SELECT</literal>
     privileges on the target table.  <literal>LOCK TABLE ... IN ROW EXCLUSIVE
     MODE</literal> requires <literal>INSERT</literal>, <literal>UPDATE</literal>, <literal>DELETE</literal>,
     or <literal>TRUNCATE</literal> privileges on the target table. All other forms of
     <command>LOCK</command> require table-level <literal>UPDATE</literal>, <literal>DELETE</literal>,
     or <literal>TRUNCATE</literal> privileges.
    </para>

    <para>
     The user performing the lock on the view must have the corresponding privilege
     on the view.  In addition the view's owner must have the relevant privileges on
     the underlying base relations, but the user performing the lock does
     not need any permissions on the underlying base relations.
    </para>

    <para>
     <command>LOCK TABLE</command> is useless outside a transaction block: the lock
     would remain held only to the completion of the statement.  Therefore
     <productname>PostgreSQL</productname> reports an error if <command>LOCK</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>
    <command>LOCK TABLE</command> only deals with table-level locks, and so
    the mode names involving <literal>ROW</literal> are all misnomers.  These
    mode names should generally be read as indicating the intention of
    the user to acquire row-level locks within the locked table.  Also,
    <literal>ROW EXCLUSIVE</literal> mode is a shareable table lock.  Keep in
    mind that all the lock modes have identical semantics so far as
    <command>LOCK TABLE</command> is concerned, differing only in the rules
    about which modes conflict with which. For information on how to
    acquire an actual row-level lock, see <xref linkend="locking-rows"/>
    and <xref linkend="sql-for-update-share"/>
    in the <xref linkend="sql-select"/> documentation.
   </para>
  </refsect1>

  <refsect1>
   <title>Examples</title>

   <para>
    Obtain a <literal>SHARE</literal> lock on a primary key table when going to perform
    inserts into a foreign key table:

 <programlisting>
 BEGIN WORK;
 LOCK TABLE films IN SHARE MODE;
 SELECT id FROM films
     WHERE name = 'Star Wars: Episode I - The Phantom Menace';
 -- Do ROLLBACK if record was not returned
 INSERT INTO films_user_comments VALUES
     (_id_, 'GREAT! I was waiting for it for so long!');
 COMMIT WORK;
 </programlisting>
   </para>

   <para>
    Take a <literal>SHARE ROW EXCLUSIVE</literal> lock on a primary key table when going to perform
    a delete operation:

 <programlisting>
 BEGIN WORK;
 LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
 DELETE FROM films_user_comments WHERE id IN
     (SELECT id FROM films WHERE rating &lt; 5);
 DELETE FROM films WHERE rating &lt; 5;
 COMMIT WORK;
 </programlisting></para>
  </refsect1>

  <refsect1>
   <title>Compatibility</title>

   <para>
    There is no <command>LOCK TABLE</command> in the SQL standard,
    which instead uses <command>SET TRANSACTION</command> to specify
    concurrency levels on transactions.  <productname>PostgreSQL</productname> supports that too;
    see <xref linkend="sql-set-transaction"/> for details.
   </para>

   <para>
    Except for <literal>ACCESS SHARE</literal>, <literal>ACCESS EXCLUSIVE</literal>,
    and <literal>SHARE UPDATE EXCLUSIVE</literal> lock modes, the
    <productname>PostgreSQL</productname> lock modes and the
    <command>LOCK TABLE</command> syntax are compatible with those
    present in <productname>Oracle</productname>.
   </para>
  </refsect1>
 </refentry>
	<!--
	doc/src/sgml/ref/lock.sgml
	PostgreSQL documentation
	-->

	<refentry id="sql-lock">
	<indexterm zone="sql-lock">
	<primary>LOCK</primary>
	</indexterm>

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

	<refnamediv>
	<refname>LOCK</refname>
	<refpurpose>lock a table</refpurpose>
	</refnamediv>

	<refsynopsisdiv>
	<synopsis>
	LOCK [ TABLE ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ] [, ...] [ IN <replaceable class="parameter">lockmode</replaceable> MODE ] [ NOWAIT ]

	<phrase>where <replaceable class="parameter">lockmode</replaceable> is one of:</phrase>

	ACCESS SHARE \| ROW SHARE \| ROW EXCLUSIVE \| SHARE UPDATE EXCLUSIVE
	\| SHARE \| SHARE ROW EXCLUSIVE \| EXCLUSIVE \| ACCESS EXCLUSIVE
	</synopsis>
	</refsynopsisdiv>

	<refsect1>
	<title>Description</title>

	<para>
	<command>LOCK TABLE</command> obtains a table-level lock, waiting
	if necessary for any conflicting locks to be released. If
	<literal>NOWAIT</literal> is specified, <command>LOCK
	TABLE</command> does not wait to acquire the desired lock: if it
	cannot be acquired immediately, the command is aborted and an
	error is emitted. Once obtained, the lock is held for the
	remainder of the current transaction. (There is no <command>UNLOCK
	TABLE</command> command; locks are always released at transaction
	end.)
	</para>

	<para>
	When a view is locked, all relations appearing in the view definition
	query are also locked recursively with the same lock mode.
	</para>

	<para>
	When acquiring locks automatically for commands that reference
	tables, <productname>PostgreSQL</productname> always uses the least
	restrictive lock mode possible. <command>LOCK TABLE</command>
	provides for cases when you might need more restrictive locking.
	For example, suppose an application runs a transaction at the
	<literal>READ COMMITTED</literal> isolation level and needs to ensure that
	data in a table remains stable for the duration of the transaction.
	To achieve this you could obtain <literal>SHARE</literal> lock mode over the
	table before querying. This will prevent concurrent data changes
	and ensure subsequent reads of the table see a stable view of
	committed data, because <literal>SHARE</literal> lock mode conflicts with
	the <literal>ROW EXCLUSIVE</literal> lock acquired by writers, and your
	<command>LOCK TABLE <replaceable
	class="parameter">name</replaceable> IN SHARE MODE</command>
	statement will wait until any concurrent holders of <literal>ROW
	EXCLUSIVE</literal> mode locks commit or roll back. Thus, once you
	obtain the lock, there are no uncommitted writes outstanding;
	furthermore none can begin until you release the lock.
	</para>

	<para>
	To achieve a similar effect when running a transaction at the
	<literal>REPEATABLE READ</literal> or <literal>SERIALIZABLE</literal>
	isolation level, you have to execute the <command>LOCK TABLE</command> statement
	before executing any <command>SELECT</command> or data modification statement.
	A <literal>REPEATABLE READ</literal> or <literal>SERIALIZABLE</literal> transaction's
	view of data will be frozen when its first
	<command>SELECT</command> or data modification statement begins. A <command>LOCK
	TABLE</command> later in the transaction will still prevent concurrent writes
	— but it won't ensure that what the transaction reads corresponds to
	the latest committed values.
	</para>

	<para>
	If a transaction of this sort is going to change the data in the
	table, then it should use <literal>SHARE ROW EXCLUSIVE</literal> lock mode
	instead of <literal>SHARE</literal> mode. This ensures that only one
	transaction of this type runs at a time. Without this, a deadlock
	is possible: two transactions might both acquire <literal>SHARE</literal>
	mode, and then be unable to also acquire <literal>ROW EXCLUSIVE</literal>
	mode to actually perform their updates. (Note that a transaction's
	own locks never conflict, so a transaction can acquire <literal>ROW
	EXCLUSIVE</literal> mode when it holds <literal>SHARE</literal> mode — but not
	if anyone else holds <literal>SHARE</literal> mode.) To avoid deadlocks,
	make sure all transactions acquire locks on the same objects in the
	same order, and if multiple lock modes are involved for a single
	object, then transactions should always acquire the most
	restrictive mode first.
	</para>

	<para>
	More information about the lock modes and locking strategies can be
	found in <xref linkend="explicit-locking"/>.
	</para>
	</refsect1>

	<refsect1>
	<title>Parameters</title>

	<variablelist>
	<varlistentry>
	<term><replaceable class="parameter">name</replaceable></term>
	<listitem>
	<para>
	The name (optionally schema-qualified) of an existing table to
	lock. If <literal>ONLY</literal> is specified before the table name, only that
	table is locked. If <literal>ONLY</literal> is not specified, the table and all
	its descendant tables (if any) are locked. Optionally, <literal>*</literal>
	can be specified after the table name to explicitly indicate that
	descendant tables are included.
	</para>

	<para>
	The command <literal>LOCK TABLE a, b;</literal> is equivalent to
	<literal>LOCK TABLE a; LOCK TABLE b;</literal>. The tables are locked
	one-by-one in the order specified in the <command>LOCK
	TABLE</command> command.
	</para>
	</listitem>
	</varlistentry>

	<varlistentry>
	<term><replaceable class="parameter">lockmode</replaceable></term>
	<listitem>
	<para>
	The lock mode specifies which locks this lock conflicts with.
	Lock modes are described in <xref linkend="explicit-locking"/>.
	</para>

	<para>
	If no lock mode is specified, then <literal>ACCESS
	EXCLUSIVE</literal>, the most restrictive mode, is used.
	</para>
	</listitem>
	</varlistentry>

	<varlistentry>
	<term><literal>NOWAIT</literal></term>
	<listitem>
	<para>
	Specifies that <command>LOCK TABLE</command> should not wait for
	any conflicting locks to be released: if the specified lock(s)
	cannot be acquired immediately without waiting, the transaction
	is aborted.
	</para>
	</listitem>
	</varlistentry>
	</variablelist>
	</refsect1>

	<refsect1>
	<title>Notes</title>

	<para>
	<literal>LOCK TABLE ... IN ACCESS SHARE MODE</literal> requires <literal>SELECT</literal>
	privileges on the target table. <literal>LOCK TABLE ... IN ROW EXCLUSIVE
	MODE</literal> requires <literal>INSERT</literal>, <literal>UPDATE</literal>, <literal>DELETE</literal>,
	or <literal>TRUNCATE</literal> privileges on the target table. All other forms of
	<command>LOCK</command> require table-level <literal>UPDATE</literal>, <literal>DELETE</literal>,
	or <literal>TRUNCATE</literal> privileges.
	</para>

	<para>
	The user performing the lock on the view must have the corresponding privilege
	on the view. In addition the view's owner must have the relevant privileges on
	the underlying base relations, but the user performing the lock does
	not need any permissions on the underlying base relations.
	</para>

	<para>
	<command>LOCK TABLE</command> is useless outside a transaction block: the lock
	would remain held only to the completion of the statement. Therefore
	<productname>PostgreSQL</productname> reports an error if <command>LOCK</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>
	<command>LOCK TABLE</command> only deals with table-level locks, and so
	the mode names involving <literal>ROW</literal> are all misnomers. These
	mode names should generally be read as indicating the intention of
	the user to acquire row-level locks within the locked table. Also,
	<literal>ROW EXCLUSIVE</literal> mode is a shareable table lock. Keep in
	mind that all the lock modes have identical semantics so far as
	<command>LOCK TABLE</command> is concerned, differing only in the rules
	about which modes conflict with which. For information on how to
	acquire an actual row-level lock, see <xref linkend="locking-rows"/>
	and <xref linkend="sql-for-update-share"/>
	in the <xref linkend="sql-select"/> documentation.
	</para>
	</refsect1>

	<refsect1>
	<title>Examples</title>

	<para>
	Obtain a <literal>SHARE</literal> lock on a primary key table when going to perform
	inserts into a foreign key table:

	<programlisting>
	BEGIN WORK;
	LOCK TABLE films IN SHARE MODE;
	SELECT id FROM films
	WHERE name = 'Star Wars: Episode I - The Phantom Menace';
	-- Do ROLLBACK if record was not returned
	INSERT INTO films_user_comments VALUES
	(_id_, 'GREAT! I was waiting for it for so long!');
	COMMIT WORK;
	</programlisting>
	</para>

	<para>
	Take a <literal>SHARE ROW EXCLUSIVE</literal> lock on a primary key table when going to perform
	a delete operation:

	<programlisting>
	BEGIN WORK;
	LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
	DELETE FROM films_user_comments WHERE id IN
	(SELECT id FROM films WHERE rating < 5);
	DELETE FROM films WHERE rating < 5;
	COMMIT WORK;
	</programlisting></para>
	</refsect1>

	<refsect1>
	<title>Compatibility</title>

	<para>
	There is no <command>LOCK TABLE</command> in the SQL standard,
	which instead uses <command>SET TRANSACTION</command> to specify
	concurrency levels on transactions. <productname>PostgreSQL</productname> supports that too;
	see <xref linkend="sql-set-transaction"/> for details.
	</para>

	<para>
	Except for <literal>ACCESS SHARE</literal>, <literal>ACCESS EXCLUSIVE</literal>,
	and <literal>SHARE UPDATE EXCLUSIVE</literal> lock modes, the
	<productname>PostgreSQL</productname> lock modes and the
	<command>LOCK TABLE</command> syntax are compatible with those
	present in <productname>Oracle</productname>.
	</para>
	</refsect1>
	</refentry>