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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_constraints.sgml,v 1.5 2002/08/17 12:15:48 momjian Exp $ -->
<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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