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98 lines
3.5 KiB
Plaintext
98 lines
3.5 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/set_constraints.sgml,v 1.14 2006/09/16 00:30:20 momjian Exp $ -->
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<refentry id="SQL-SET-CONSTRAINTS">
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<refmeta>
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<refentrytitle id="SQL-SET-CONSTRAINTS-title">SET CONSTRAINTS</refentrytitle>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>SET CONSTRAINTS</refname>
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<refpurpose>set constraint checking modes for the current transaction</refpurpose>
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</refnamediv>
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<indexterm zone="sql-set-constraints">
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<primary>SET CONSTRAINTS</primary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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SET CONSTRAINTS { ALL | <replaceable class="parameter">name</replaceable> [, ...] } { DEFERRED | IMMEDIATE }
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>SET CONSTRAINTS</command> sets the behavior of constraint
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checking within the current transaction. <literal>IMMEDIATE</literal>
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constraints are checked at the end of each
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statement. <literal>DEFERRED</literal> constraints are not checked until
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transaction commit. Each constraint has its own
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<literal>IMMEDIATE</literal> or <literal>DEFERRED</literal> mode.
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</para>
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<para>
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Upon creation, a constraint is given one of three
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characteristics: <literal>DEFERRABLE INITIALLY DEFERRED</literal>,
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<literal>DEFERRABLE INITIALLY IMMEDIATE</literal>, or
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<literal>NOT DEFERRABLE</literal>. The third
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class is always <literal>IMMEDIATE</literal> and is not affected by the
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<command>SET CONSTRAINTS</command> command. The first two classes start
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every transaction in the indicated mode, but their behavior can be changed
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within a transaction by <command>SET CONSTRAINTS</command>.
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</para>
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<para>
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<command>SET CONSTRAINTS</command> with a list of constraint names changes
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the mode of just those constraints (which must all be deferrable). The
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current schema search path is used to find the first matching name if
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no schema name is specified. <command>SET CONSTRAINTS ALL</command>
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changes the mode of all deferrable constraints.
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</para>
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<para>
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When <command>SET CONSTRAINTS</command> changes the mode of a constraint
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from <literal>DEFERRED</literal>
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to <literal>IMMEDIATE</literal>, the new mode takes effect
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retroactively: any outstanding data modifications that would have
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been checked at the end of the transaction are instead checked during the
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execution of the <command>SET CONSTRAINTS</command> command.
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If any such constraint is violated, the <command>SET CONSTRAINTS</command>
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fails (and does not change the constraint mode). Thus, <command>SET
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CONSTRAINTS</command> can be used to force checking of constraints to
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occur at a specific point in a transaction.
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</para>
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<para>
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Currently, only foreign key constraints are affected by this
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setting. Check and unique constraints are always effectively
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not deferrable.
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</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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This command only alters the behavior of constraints within the
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current transaction. Thus, if you execute this command outside of a
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transaction block
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(<command>BEGIN</command>/<command>COMMIT</command> pair), it will
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not appear to have any effect.
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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This command complies with the behavior defined in the SQL
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standard, except for the limitation that, in
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<productname>PostgreSQL</productname>, it only applies to
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foreign-key constraints.
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</para>
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</refsect1>
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</refentry>
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