<refentry id="SQL-LOCK">
 <refmeta>
  <refentrytitle>
   LOCK
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   LOCK
  </refname>
  <refpurpose>
   Explicit lock of a table inside a transaction
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1998-09-24</date>
  </refsynopsisdivinfo>
  <synopsis>
LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable>
LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN [ ROW | ACCESS ] { SHARE | EXCLUSIVE } MODE
LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW EXCLUSIVE MODE
  </synopsis>

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

    <variablelist>
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">table</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.
	<productname>Postgres</productname> releases automatically acquired
	ACCESS SHARE locks after the statement is done.
	   </para>
	   </note>

       <para>
	This is the least restrictive lock mode which conflicts only with 
	ACCESS EXCLUSIVE mode. It is intended to protect a table being
	queried from concurrent <command>ALTER TABLE</command>, 
	<command>DROP TABLE</command> and <command>VACUUM</command> 
	statements over the same table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       ROW SHARE MODE
      </term>
      <listitem>
       <note>
       <para>
	 Automatically acquired by any <command>SELECT FOR UPDATE</command> statement.
	</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 any <command>UPDATE</command>, 
	 <command>DELETE</command>, <command>INSERT</command> statement.
	</para>
       </note>

       <para>
	 Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and
	 ACCESS EXCLUSIVE modes. Generally means that a transaction
	 updated or inserted some tuples in a table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       SHARE MODE
      </term>
      <listitem>
       <note>
       <para>
	 Automatically acquired by any <command>CREATE INDEX</command> statement.
       </para>
       </note>

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

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

       <para>
	Conflicts with ROW EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE,
	EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode is more
	restrictive than SHARE mode because of only one transaction
	at time can hold this lock.
       </para>
      </listitem>
     </varlistentry>

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

       <para>
	Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE,
	EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode is yet more
	restrictive than that of SHARE ROW EXCLUSIVE; it blocks all concurrent
	SELECT FOR UPDATE queries.
       </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</command> statements.
	</para>
       </note>

       <para>
	This is the most restrictive lock mode which conflicts with all other
	lock modes and protects a locked table from any concurrent operations.
       </para>
	   
       <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>
      </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>
       ERROR <replaceable class="PARAMETER">table</replaceable>: Table does not exist.
      </term>
      <listitem>
       <para>
	Message returned if <replaceable class="PARAMETER">table</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>
   <productname>Postgres</productname> always uses the least restrictive
   lock mode whenever possible. <command>LOCK TABLE</command> 
   provided for cases when you might need more restrictive locking.
  </para>

  <para>
   For example, an application runs a transaction at READ COMMITTED isolation
   level and needs to ensure the existance 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 LOCK TABLE table IN SHARE MODE 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 execution 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 concurrent SHARE lock.
  </para>
  
  <para>
   To continue with the deadlock (when two transaction wait 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/EXCLUSIVE lock modes, all other
    <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>
  <programlisting>
    --
    -- SHARE lock primary key table when going to perform
    -- insert into foreign key table.
    --
    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>
  <programlisting>
    --
    -- SHARE ROW EXCLUSIVE lock primary key table when going to perform
    -- delete operation.
    --
    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 level on transactions.  We support that too; see
    <xref linkend="SQL-SET-TITLE" endterm="SQL-SET-TITLE"> for details.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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