<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/lock.sgml,v 1.34 2002/09/21 18:32:54 petere Exp $
PostgreSQL documentation
-->

<refentry id="SQL-LOCK">
 <refmeta>
  <refentrytitle id="sql-lock-title">LOCK</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   LOCK
  </refname>
  <refpurpose>
   explicitly lock a table
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2001-07-09</date>
  </refsynopsisdivinfo>
  <synopsis>
LOCK [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...]
LOCK [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...] IN <replaceable class="PARAMETER">lockmode</replaceable> MODE

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

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

  <refsect2 id="R2-SQL-LOCK-1">
   <refsect2info>
    <date>1999-06-09</date>
   </refsect2info>

   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER">name</replaceable></term>
      <listitem>
       <para>
	The name (optionally schema-qualified) of an existing table to lock.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ACCESS SHARE MODE</term>
      <listitem>
       <para>
	This is the least restrictive lock mode.  It conflicts only with
	ACCESS EXCLUSIVE mode. It is used to protect a table from being
	modified by concurrent <command>ALTER TABLE</command>, 
	<command>DROP TABLE</command> and <command>VACUUM FULL</command>
	commands.
       </para>

       <note>
	<para>
	 The <command>SELECT</command> command acquires a
	 lock of this mode on referenced tables.  In general, any query
	 that only reads a table and does not modify it will acquire
	 this lock mode.
	</para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ROW SHARE MODE</term>
      <listitem>
       <para>
	Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock modes.
       </para>

       <note>
       <para>
	 The <command>SELECT FOR UPDATE</command> command acquires a
	 lock of this mode on the target table(s) (in addition to
	 <literal>ACCESS SHARE</literal> locks on any other tables
	 that are referenced but not selected <option>FOR UPDATE</option>).
	</para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ROW EXCLUSIVE MODE</term>
      <listitem>
       <para>
	 Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and
	 ACCESS EXCLUSIVE modes.
       </para>

       <note>
	<para>
	 The commands <command>UPDATE</command>,
	 <command>DELETE</command>, and <command>INSERT</command>
	 acquire this lock mode on the target table (in addition to
	 <literal>ACCESS SHARE</literal> locks on any other referenced
	 tables).  In general, this lock mode will be acquired by any
	 query that modifies the data in a table.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SHARE UPDATE EXCLUSIVE MODE</term>
      <listitem>
       <para>
	Conflicts with SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE,
	EXCLUSIVE and
	ACCESS EXCLUSIVE modes. This mode protects a table against
	concurrent schema changes and <command>VACUUM</> runs.
       </para>

       <note>
       <para>
	 Acquired by <command>VACUUM</command> (without
	 <option>FULL</option>).
       </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SHARE MODE</term>
      <listitem>
       <para>
	Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE,
	SHARE ROW EXCLUSIVE, EXCLUSIVE and
	ACCESS EXCLUSIVE modes. This mode protects a table against
	concurrent data changes.
       </para>

       <note>
       <para>
	 Acquired by <command>CREATE INDEX</command>.
       </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SHARE ROW EXCLUSIVE MODE</term>
      <listitem>
       <para>
        Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE,
	SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes.
       </para>

       <note>
       <para>
         This lock mode is not automatically acquired by any
         <productname>PostgreSQL</productname> command.
       </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>EXCLUSIVE MODE</term>
      <listitem>
       <para>
	Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE,
	SHARE, SHARE ROW EXCLUSIVE,
	EXCLUSIVE and ACCESS EXCLUSIVE modes.
	This mode allows only concurrent ACCESS SHARE, i.e., only reads
	from the table can proceed in parallel with a transaction holding
	this lock mode.
       </para>

       <note>
       <para>
        This lock mode is not automatically acquired by any
        <productname>PostgreSQL</productname> command.
       </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ACCESS EXCLUSIVE MODE</term>
      <listitem>
       <para>
	Conflicts with all lock modes.  This mode guarantees that the
	holder is the only transaction accessing the table in any way.
       </para>

       <note>
	<para>
	 Acquired by <command>ALTER TABLE</command>,
	 <command>DROP TABLE</command>, and <command>VACUUM FULL</command>
	 statements.
	 This is also the default lock mode for <command>LOCK TABLE</command>
         statements that do not specify a mode explicitly.
	</para>
       </note>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-LOCK-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>
    
    <variablelist>
     <varlistentry>
      <term><computeroutput>
LOCK TABLE
       </computeroutput></term>
      <listitem>
       <para>
	The lock was successfully acquired.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
       </computeroutput></term>
      <listitem>
       <para>
	Message returned if <replaceable class="PARAMETER">name</replaceable>
	does not exist.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-LOCK-1">
  <refsect1info>
   <date>1998-09-24</date>
  </refsect1info>
  <title>
   Description
  </title>

  <para>
   <command>LOCK TABLE</command> obtains a table-level lock, waiting if
   necessary for any conflicting locks to be released.  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 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.
  </para>

  <para>
   For example, suppose an application runs a transaction at READ COMMITTED
   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 SHARE 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 SHARE lock mode conflicts with the ROW
   EXCLUSIVE 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 ROW EXCLUSIVE mode
   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.

   <note>
    <para>
     To achieve a similar effect when running a transaction
     at the SERIALIZABLE isolation level, you have to execute the
     <command>LOCK TABLE</>
     statement before executing any <acronym>DML</> statement.  A serializable
     transaction's view of data will be frozen when its first <acronym>DML</> statement
     begins.  A later <command>LOCK</> will still prevent concurrent writes
     --- but it
     won't ensure that what the transaction reads corresponds to the latest
     committed values.
    </para>
   </note>
  </para>
  
  <para>
   If a transaction of this sort is going to
   change the data in the table, then it should use SHARE ROW EXCLUSIVE lock
   mode instead of SHARE 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 SHARE mode, and then be unable to also
   acquire ROW EXCLUSIVE mode to actually perform their updates.  (Note that
   a transaction's own locks never conflict, so a transaction can acquire
   ROW EXCLUSIVE mode when it holds SHARE mode --- but not if anyone else
   holds SHARE mode.)
  </para>
  
  <para>
   Two general rules may be followed to prevent deadlock conditions:
  </para>

  <itemizedlist>  
   <listitem>
    <para>
     Transactions have to acquire locks on the same objects in the same order.
    </para>
   
    <para>
     For example, if one application updates row R1 and than updates 
     row R2 (in the same transaction) then the second application shouldn't 
     update row R2 if it's going to update row R1 later (in a single transaction). 
     Instead, it should update rows R1 and R2 in the same order as the first 
     application.
    </para>
   </listitem>

   <listitem>
    <para>
     If multiple lock modes are involved for a single object,
     then transactions should always acquire the  most restrictive mode first.
    </para>
   
    <para>
     An example for this rule was given previously when discussing the 
     use of SHARE ROW EXCLUSIVE mode rather than SHARE mode.
    </para>

   </listitem>
  </itemizedlist>

  <para>
   <productname>PostgreSQL</productname> does detect deadlocks and will
   rollback at least one waiting transaction to resolve the deadlock.
   If it is not practical to code an application to follow the above rules
   strictly, an alternative solution is to be prepared to retry transactions
   when they are aborted by deadlocks.
  </para>

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

  <refsect2 id="R2-SQL-LOCK-3">
   <refsect2info>
    <date>1999-06-08</date>
   </refsect2info>
   <title>
    Notes
   </title>

   <para>
    <literal>LOCK ... IN ACCESS SHARE MODE</> requires <literal>SELECT</>
    privileges on the target table.  All other forms of <command>LOCK</>
    require <literal>UPDATE</> and/or <literal>DELETE</> privileges.
   </para>

   <para>
    <command>LOCK</command> is useful only inside a transaction block
    (<command>BEGIN</>...<command>COMMIT</>), since the lock is dropped
    as soon as the transaction ends.  A <command>LOCK</> command appearing
    outside any transaction block forms a self-contained transaction, so the
    lock will be dropped as soon as it is obtained.
   </para>

  <para>
   <acronym>RDBMS</acronym> locking uses the following standard terminology:

   <variablelist>
    <varlistentry>
     <term>EXCLUSIVE</term>
     <listitem>
      <para>
       An exclusive lock prevents other locks of the same type from being
       granted.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>SHARE</term>
     <listitem>
      <para>
      A shared lock allows others to also hold the same type of lock,
      but prevents the corresponding EXCLUSIVE lock from being granted.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>ACCESS</term>
     <listitem>
      <para>
       Locks table schema.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>ROW</term>
     <listitem>
      <para>
       Locks individual rows.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   <productname>PostgreSQL</productname> does not follow this terminology
   exactly.  <command>LOCK TABLE</> only deals with table-level locks, and
   so the mode names involving ROW 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,
   ROW EXCLUSIVE mode does not follow this naming convention accurately,
   since it is a sharable table lock.  Keep in mind that all the lock modes
   have identical semantics so far as <command>LOCK TABLE</> is concerned,
   differing only in the rules about which modes conflict with which.
  </para>

  </refsect2>
 </refsect1>
  
 <refsect1 id="R1-SQL-LOCK-2">
  <title>
   Usage
  </title>

  <para>
   Obtain a SHARE 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 SHARE ROW EXCLUSIVE 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 id="R1-SQL-LOCK-3">
  <title>
   Compatibility
  </title>
	  
  <refsect2 id="R2-SQL-LOCK-4">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no <command>LOCK TABLE</command> in <acronym>SQL92</acronym>,
    which instead uses <command>SET TRANSACTION</command> to specify
    concurrency levels on transactions.  We support that too; see
    <xref linkend="SQL-SET-TRANSACTION" endterm="SQL-SET-TRANSACTION-TITLE"> for details.
   </para>

   <para>
    Except for ACCESS SHARE, ACCESS EXCLUSIVE, and SHARE UPDATE EXCLUSIVE lock
    modes, the <productname>PostgreSQL</productname> lock modes and the
    <command>LOCK TABLE</command> syntax are compatible with those
    present in <productname>Oracle</productname>(TM).
   </para>
  </refsect2>
 </refsect1>
</refentry>

<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"../reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:"/usr/lib/sgml/catalog"
sgml-local-ecat-files:nil
End:
-->
