<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/set_constraints.sgml,v 1.17 2009/07/29 20:56:17 tgl Exp $ -->
<refentry id="SQL-SET-CONSTRAINTS">
 <refmeta>
  <refentrytitle id="SQL-SET-CONSTRAINTS-title">SET CONSTRAINTS</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>SET CONSTRAINTS</refname>
  <refpurpose>set constraint checking modes for the current transaction</refpurpose>
 </refnamediv>

 <indexterm zone="sql-set-constraints">
  <primary>SET CONSTRAINTS</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
SET CONSTRAINTS { ALL | <replaceable class="parameter">name</replaceable> [, ...] } { DEFERRED | IMMEDIATE }
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>SET CONSTRAINTS</command> sets the behavior of constraint
   checking within the current transaction. <literal>IMMEDIATE</literal>
   constraints are checked at the end of each
   statement. <literal>DEFERRED</literal> constraints are not checked until
   transaction commit.  Each constraint has its own
   <literal>IMMEDIATE</literal> or <literal>DEFERRED</literal> mode.
  </para>

  <para>
   Upon creation, a constraint is given one of three
   characteristics: <literal>DEFERRABLE INITIALLY DEFERRED</literal>,
   <literal>DEFERRABLE INITIALLY IMMEDIATE</literal>, or
   <literal>NOT DEFERRABLE</literal>. The third
   class is always <literal>IMMEDIATE</literal> and is not affected by the
   <command>SET CONSTRAINTS</command> command.  The first two classes start
   every transaction in the indicated mode, but their behavior can be changed
   within a transaction by <command>SET CONSTRAINTS</command>.
  </para>

  <para>
   <command>SET CONSTRAINTS</command> with a list of constraint names changes
   the mode of just those constraints (which must all be deferrable).  The
   current schema search path is used to find the first matching name if
   no schema name is specified.  <command>SET CONSTRAINTS ALL</command>
   changes the mode of all deferrable constraints.
  </para>

  <para>
   When <command>SET CONSTRAINTS</command> changes the mode of a constraint
   from <literal>DEFERRED</literal>
   to <literal>IMMEDIATE</literal>, the new mode takes effect
   retroactively: any outstanding data modifications that would have
   been checked at the end of the transaction are instead checked during the
   execution of the <command>SET CONSTRAINTS</command> command.
   If any such constraint is violated, the <command>SET CONSTRAINTS</command>
   fails (and does not change the constraint mode).  Thus, <command>SET
   CONSTRAINTS</command> can be used to force checking of constraints to
   occur at a specific point in a transaction.
  </para>

  <para>
   Currently, only <literal>UNIQUE</>, <literal>PRIMARY KEY</>, and
   <literal>REFERENCES</> (foreign key) constraints are affected by this
   setting.  <literal>NOT NULL</> and <literal>CHECK</> constraints are
   always checked immediately when a row is inserted or modified
   (<emphasis>not</> at the end of the statement).
   Uniqueness constraints that have not been declared <literal>DEFERRABLE</>
   are also checked immediately.
  </para>

  <para>
   The firing of triggers that are declared as <quote>constraint triggers</>
   is also controlled by this setting &mdash; they fire at the same time
   that the associated constraint should be checked.
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   This command only alters the behavior of constraints within the
   current transaction. Thus, if you execute this command outside of a
   transaction block
   (<command>BEGIN</command>/<command>COMMIT</command> pair), it will
   not appear to have any effect.
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   This command complies with the behavior defined in the SQL
   standard, except for the limitation that, in
   <productname>PostgreSQL</productname>, it only applies to
   foreign-key and uniqueness constraints.
  </para>

 </refsect1>
</refentry>
