path: root/doc/src/sgml/ref/lock.sgml

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doc/src/sgml/ref/lock.sgml
PostgreSQL documentation
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<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>
    To lock a table, the user must have the right privilege for the specified
    <replaceable class="parameter">lockmode</replaceable>, or be the table's
    owner or a superuser.  If the user has
    <literal>UPDATE</literal>, <literal>DELETE</literal>, or
    <literal>TRUNCATE</literal> privileges on the table, any <replaceable
    class="parameter">lockmode</replaceable> is permitted. If the user has
    <literal>INSERT</literal> privileges on the table, <literal>ROW EXCLUSIVE
    MODE</literal> (or a less-conflicting mode as described in <xref
    linkend="explicit-locking"/>) is permitted. If a user has
    <literal>SELECT</literal> privileges on the table, <literal>ACCESS SHARE
    MODE</literal> is permitted.
   </para>

   <para>
    The user performing the lock on the view must have the corresponding
    privilege on the view.  In addition, by default, the view's owner must
    have the relevant privileges on the underlying base relations, whereas the
    user performing the lock does not need any permissions on the underlying
    base relations.  However, if the view has
    <literal>security_invoker</literal> set to <literal>true</literal>
    (see <link linkend="sql-createview"><command>CREATE VIEW</command></link>),
    the user performing the lock, rather than the view owner, must have the
    relevant privileges 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>