126 lines
4.7 KiB
Plaintext
126 lines
4.7 KiB
Plaintext
<!--
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doc/src/sgml/ref/set_constraints.sgml
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PostgreSQL documentation
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-->
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<refentry id="sql-set-constraints">
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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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<refmeta>
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<refentrytitle>SET CONSTRAINTS</refentrytitle>
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<manvolnum>7</manvolnum>
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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 check timing for the current transaction</refpurpose>
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</refnamediv>
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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). Each
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constraint name can be schema-qualified. 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 <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>,
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<literal>REFERENCES</literal> (foreign key), and <literal>EXCLUDE</literal>
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constraints are affected by this setting.
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<literal>NOT NULL</literal> and <literal>CHECK</literal> constraints are
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always checked immediately when a row is inserted or modified
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(<emphasis>not</emphasis> at the end of the statement).
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Uniqueness and exclusion constraints that have not been declared
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<literal>DEFERRABLE</literal> are also checked immediately.
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</para>
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<para>
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The firing of triggers that are declared as <quote>constraint triggers</quote>
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is also controlled by this setting — they fire at the same time
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that the associated constraint should be checked.
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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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Because <productname>PostgreSQL</productname> does not require constraint
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names to be unique within a schema (but only per-table), it is possible
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that there is more than one match for a specified constraint name.
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In this case <command>SET CONSTRAINTS</command> will act on all matches.
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For a non-schema-qualified name, once a match or matches have been found in
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some schema in the search path, schemas appearing later in the path are not
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searched.
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</para>
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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. Issuing this outside of a transaction block
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emits a warning and otherwise has no 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 does not apply to
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<literal>NOT NULL</literal> and <literal>CHECK</literal> constraints.
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Also, <productname>PostgreSQL</productname> checks non-deferrable
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uniqueness constraints immediately, not at end of statement as the
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standard would suggest.
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</para>
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</refsect1>
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</refentry>
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