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Postgres documentation

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<term>CONSTRAINT</term>

<term>CONSTRAINTS</term>

<listitem>

<para>

SET CONSTRAINTS controls the frequency of foreign key

constratint checking in the current transaction. Allowed

parameters are:

SET CONSTRAINTS affects the behavior of constraint evaluation

in the current transaction.

SET CONSTRAINTS, specified

in SQL3, has these allowed parameters:

<variablelist>

<varlistentry>

<term><replaceable class="parameter">constraintlist</replaceable></term>

</para>

<para>

In <option>DEFERRED</option> mode, foreign key constraints

marked as <option>DEFERRABLE</option> are checked only at

transaction commit.

In <option>IMMEDIATE</option> mode, foreign key constraints

are checked at the end of each query.

</para>

<para>

In <option>DEFERRED</option> mode, foreign key constraints

marked as <option>DEFERRABLE</option> are checked only at

transaction commit or until its mode is explicitly set to

<option>IMMEDIATE</option>.

This is actually only done for foreign key

constraints, so it does not apply to UNIQUE or other

constraints.

</para>

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