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$Header: /cvsroot/pgsql/doc/src/sgml/ref/lock.sgml,v 1.24 2001/07/09 22:18:33 tgl Exp $
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<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 inside a transaction
  </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 of an existing table to lock.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ACCESS SHARE MODE</term>
      <listitem>
       <note>
	<para>
	 This lock mode is acquired automatically over tables being queried.
	</para>
       </note>

       <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>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ROW SHARE MODE</term>
      <listitem>
       <note>
       <para>
	 Automatically acquired by <command>SELECT...FOR UPDATE</command>.
	 While it is a shared lock, may be upgraded later to a ROW EXCLUSIVE lock.
	</para>
       </note>

       <para>
	Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock modes.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>ROW EXCLUSIVE MODE</term>
      <listitem>
       <note>
	<para>
	 Automatically acquired by <command>UPDATE</command>, 
	 <command>DELETE</command>, and <command>INSERT</command>
	 statements.
        </para>
       </note>

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

     <varlistentry>
      <term>SHARE UPDATE EXCLUSIVE MODE</term>
      <listitem>
       <note>
       <para>
	 Automatically acquired by <command>VACUUM</command> (without
	 <option>FULL</option>).
       </para>
       </note>

       <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 VACUUMs.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>SHARE MODE</term>
      <listitem>
       <note>
       <para>
	 Automatically acquired by <command>CREATE INDEX</command>.
	 Share-locks the entire table.
       </para>
       </note>

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

     <varlistentry>
      <term>SHARE ROW EXCLUSIVE MODE</term>
      <listitem>
       <note>
       <para>
	This is like EXCLUSIVE MODE, but allows ROW SHARE locks
        by others.
       </para>
       </note>

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

     <varlistentry>
      <term>EXCLUSIVE MODE</term>
      <listitem>

       <note>
       <para>
	This mode is yet more restrictive than SHARE ROW EXCLUSIVE.  
	It blocks all concurrent ROW SHARE/SELECT...FOR UPDATE queries.
       </para>
       </note>

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

     <varlistentry>
      <term>ACCESS EXCLUSIVE MODE</term>
      <listitem>
       <note>
	<para>
	 Automatically acquired by <command>ALTER TABLE</command>,
	 <command>DROP TABLE</command>, <command>VACUUM FULL</command>
	 statements.
	 This is the most restrictive lock mode which
	 protects a locked table from any concurrent operations.
	</para>
       </note>

       <note>
	<para>
	 This lock mode is also acquired by an unqualified
	 <command>LOCK TABLE</command> (i.e., the command without an explicit
	 lock mode option).
	</para>
       </note>

       <para>
	Conflicts with all lock modes.
       </para>
      </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 applied.
       </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> controls concurrent access to a table
   for the duration of a transaction.
   <productname>Postgres</productname> always uses the least restrictive
   lock mode whenever possible. <command>LOCK TABLE</command> 
   provides for cases when you might need more restrictive locking.
  </para>

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

   <variablelist>
    <varlistentry>
     <term>EXCLUSIVE</term>
     <listitem>
      <para>
       An exclusive lock prevents other locks of the same type from being
       granted.  (Note: ROW EXCLUSIVE mode does not follow this naming
       convention perfectly, since it is shared at the level of the table;
       it is exclusive only with respect to specific rows that are being
       updated.)
      </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>
   For example, an application runs a transaction at READ COMMITTED isolation
   level and needs to ensure the existence of data in a table for the
   duration of the
   transaction. To achieve this you could use SHARE lock mode over the
   table before querying. This will protect data from concurrent changes 
   and provide any further read operations over the table with data in their 
   actual current state, because SHARE lock mode conflicts with any ROW EXCLUSIVE 
   one acquired by writers, and your
   <command>LOCK TABLE <replaceable class="PARAMETER">name</replaceable> IN SHARE MODE</command>
   statement will wait until any concurrent write operations commit or rollback.

   <note>
    <para>
     To read data in their real current state when running a transaction
     at the SERIALIZABLE isolation level you have to execute a LOCK TABLE
     statement before executing any DML statement, when the transaction defines
     what concurrent changes will be visible to itself.
    </para>
   </note>
  </para>
  
  <para>
   In addition to the requirements above, if a transaction is going to
   change data in a table, then SHARE ROW EXCLUSIVE lock mode should
   be acquired to prevent deadlock conditions when two concurrent
   transactions attempt to lock the table in SHARE mode and then
   try to change data in this table, both (implicitly) acquiring 
   ROW EXCLUSIVE lock mode that conflicts with a concurrent SHARE lock.
  </para>
  
  <para>
   To continue with the deadlock (when two transaction wait for one another)
   issue raised above, you should follow two general rules 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>
     Transactions should acquire two conflicting lock modes only if
     one of them is self-conflicting (i.e., may be held by one
     transaction at time only). If multiple lock modes are involved,
     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>

  <note>
   <para>
    <productname>Postgres</productname> does detect deadlocks and will
    rollback at least one waiting transaction to resolve the deadlock. 
   </para>
  </note>

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

   <para>
    <command>LOCK</command> is a <productname>Postgres</productname>
    language extension.
   </para>

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

   <para>
    <command>LOCK</command> works only inside transactions.
   </para>
  </refsect2>
 </refsect1>
  
 <refsect1 id="R1-SQL-LOCK-2">
  <title>
   Usage
  </title>

  <para>
   Illustrate 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>
  </refsect2>
 </refsect1>
</refentry>

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