postgresql/doc/src/sgml/ref/set_constraints.sgml

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<refentry id="SQL-SET-CONSTRAINTS">
 <refmeta>
  <refentrytitle id="SQL-SET-CONSTRAINTS-title">SET CONSTRAINTS</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>SET CONSTRAINTS</refname>
  <refpurpose>set the constraint mode of the current transaction</refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2000-06-01</date>
  </refsynopsisdivinfo>
  <synopsis>
SET CONSTRAINTS { ALL | <replaceable class="parameter">constraint</replaceable> [, ...] } { DEFERRED | IMMEDIATE }
  </synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>SET CONSTRAINTS</command> sets the behavior of constraint
   evaluation in the current transaction. In <option>IMMEDIATE
   </option> mode, constraints are checked at the end of each
   statement. In <option>DEFERRED</option> mode, constraints are not
   checked until transaction commit.
  </para>

  <note>
   <para>
	This command only alters the behavior of constraints within the
	current transaction. Thus, if you execute this command outside
	of an explicit transaction block (such as one started with
	<command>BEGIN</command>), it will not appear to have any effect.
	If you wish to change the behavior of a constraint without needing
	to issue a <command>SET CONSTRAINTS</command> command in every
	transaction, specify <option>INITIALLY DEFERRED</option> or
	<option>INITIALLY IMMEDIATE</option> when you create the constraint.
   </para>
  </note>

  <para>
   When you change the mode of a constraint to be <option>IMMEDIATE
   </option>, the new constraint mode takes effect retroactively:
   any outstanding data modifications that would have been checked
   at the end of the transaction (when using
   <option>DEFERRED</option>) are instead checked during the
   execution of the <command>SET CONSTRAINTS</command> command.
  </para>

  <para>
   Upon creation, a constraint is always give one of three
   characteristics: <option>INITIALLY DEFERRED</option>,
   <option>INITIALLY IMMEDIATE DEFERRABLE</option>, or
   <option>INITIALLY IMMEDIATE NOT DEFERRABLE</option>. The third
   class is not affected by the <command>SET CONSTRAINTS</command>
   command.
  </para>

  <para>
   Currently, only foreign key constraints are affected by this
   setting. Check and unique constraints are always effectively
   initially immediate not deferrable.
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-SET-CONSTRAINT-3">
  <title>Compatibility</title>

  <refsect2 id="R2-SQL-SET-CONSTRAINT-4">
   <title>SQL92, SQL99</title>

   <para>
    <command>SET CONSTRAINTS</command> is defined in
    <acronym>SQL92</acronym> and <acronym>SQL99</acronym>. The
    implementation in <productname>PostgreSQL</productname> complies
    with the behavior defined in the standard, except for the
    <productname>PostgreSQL</productname> limitation that <command>SET
    CONSTRAINTS</command> cannot be applied to check or unique constraints.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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