<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/set_constraints.sgml,v 1.18 2010/01/18 00:32:21 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 check timing 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).  Each
   constraint name can be schema-qualified.  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</>,
   <literal>REFERENCES</> (foreign key), and <literal>EXCLUDE</>
   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 and exclusion 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>
   Because <productname>PostgreSQL</productname> does not require constraint
   names to be unique within a schema (but only per-table), it is possible
   that there is more than one match for a specified constraint name.
   In this case <command>SET CONSTRAINTS</command> will act on all matches.
   For a non-schema-qualified name, once a match or matches have been found in
   some schema in the search path, schemas appearing later in the path are not
   searched.
  </para>

  <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 does not apply to
   <literal>NOT NULL</> and <literal>CHECK</> constraints.
   Also, <productname>PostgreSQL</productname> checks non-deferrable
   uniqueness constraints immediately, not at end of statement as the
   standard would suggest.
  </para>

 </refsect1>
</refentry>
