set_constraints.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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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 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).  If
   there are multiple constraints matching any given name, all are affected.
   <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 foreign key constraints are affected by this
   setting. Check and unique constraints are always effectively
   not deferrable.
  </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 constraints.
  </para>

  <para>
   The SQL standard says that constraint names appearing in <command>SET
   CONSTRAINTS</command> can be schema-qualified.  This is not yet
   supported by <productname>PostgreSQL</productname>: the names must
   be unqualified, and all constraints matching the command will be
   affected no matter which schema they are in.
  </para>
 </refsect1>
</refentry>

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