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including: - replacing all the appropriate usages of <citetitle>PostgreSQL ...</citetitle> with &cite-user;, &cite-admin;, and so on - fix an omission in the EXECUTE documentation - add some more text to the EXPLAIN documentation - improve the PL/PgSQL RETURN NEXT documentation (more work to do here) - minor markup fixes Neil Conway
372 lines
12 KiB
Plaintext
372 lines
12 KiB
Plaintext
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_trigger.sgml,v 1.32 2003/01/19 00:13:29 momjian Exp $
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PostgreSQL documentation
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-->
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<refentry id="SQL-CREATETRIGGER">
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<refmeta>
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<refentrytitle id="SQL-CREATETRIGGER-TITLE">CREATE TRIGGER</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>
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CREATE TRIGGER
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</refname>
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<refpurpose>
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define a new trigger
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</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<refsynopsisdivinfo>
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<date>2000-03-25</date>
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</refsynopsisdivinfo>
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<synopsis>
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CREATE TRIGGER <replaceable class="PARAMETER">name</replaceable> { BEFORE | AFTER } { <replaceable class="PARAMETER">event</replaceable> [ OR ... ] }
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ON <replaceable class="PARAMETER">table</replaceable> [ FOR [ EACH ] { ROW | STATEMENT } ]
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EXECUTE PROCEDURE <replaceable class="PARAMETER">func</replaceable> ( <replaceable class="PARAMETER">arguments</replaceable> )
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</synopsis>
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<refsect2 id="R2-SQL-CREATETRIGGER-1">
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<refsect2info>
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<date>1998-09-21</date>
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</refsect2info>
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<title>
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Inputs
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</title>
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<para>
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">name</replaceable></term>
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<listitem>
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<para>
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The name to give the new trigger. This must be distinct from the name
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of any other trigger for the same table.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>BEFORE</term>
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<term>AFTER</term>
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<listitem>
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<para>
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Determines whether the function is called before or after the
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event.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">event</replaceable></term>
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<listitem>
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<para>
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One of <command>INSERT</command>, <command>DELETE</command> or
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<command>UPDATE</command>; this specifies the event that will
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fire the trigger. Multiple events can be specified using
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<literal>OR</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">table</replaceable></term>
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<listitem>
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<para>
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The name (optionally schema-qualified) of the table the
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trigger is for.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>FOR EACH ROW</term>
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<term>FOR EACH STATEMENT</term>
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<listitem>
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<para>
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This specifies whether the trigger procedure should be fired
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once for every row affected by the trigger event, or just once
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per SQL statement. If neither is specified, <literal>FOR EACH
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STATEMENT</literal> is the default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">func</replaceable></term>
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<listitem>
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<para>
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A user-supplied function that is declared as taking no arguments
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and returning type <literal>trigger</>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">arguments</replaceable></term>
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<listitem>
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<para>
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An optional comma-separated list of arguments to be provided to
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the function when the trigger is executed, along with the standard
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trigger data such as old and new tuple contents. The arguments
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are literal string constants. Simple names and numeric constants
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may be written here too, but they will all be converted to
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strings. Note that these arguments are not provided as normal
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function parameters (since a trigger procedure must be declared to
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take zero parameters), but are instead accessed through the
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<literal>TG_ARGV</literal> array.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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<refsect2 id="R2-SQL-CREATETRIGGER-2">
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<refsect2info>
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<date>1998-09-21</date>
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</refsect2info>
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<title>
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Outputs
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</title>
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<para>
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<variablelist>
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<varlistentry>
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<term><computeroutput>
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CREATE TRIGGER
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</computeroutput></term>
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<listitem>
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<para>
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This message is returned if the trigger is successfully created.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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</refsynopsisdiv>
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<refsect1 id="R1-SQL-CREATETRIGGER-1">
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<refsect1info>
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<date>1998-09-21</date>
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</refsect1info>
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<title>
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Description
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</title>
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<para>
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<command>CREATE TRIGGER</command> will enter a new trigger into the current
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database. The trigger will be associated with the relation
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<replaceable class="parameter">table</replaceable> and will execute
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the specified function <replaceable class="parameter">func</replaceable>.
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</para>
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<para>
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The trigger can be specified to fire either before BEFORE the
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operation is attempted on a tuple (before constraints are checked and
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the <command>INSERT</command>, <command>UPDATE</command> or
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<command>DELETE</command> is attempted) or AFTER the operation has
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been attempted (e.g., after constraints are checked and the
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<command>INSERT</command>, <command>UPDATE</command> or
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<command>DELETE</command> has completed). If the trigger fires before
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the event, the trigger may skip the operation for the current tuple,
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or change the tuple being inserted (for <command>INSERT</command> and
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<command>UPDATE</command> operations only). If the trigger fires
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after the event, all changes, including the last insertion, update,
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or deletion, are <quote>visible</quote> to the trigger.
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</para>
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<para>
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A trigger that executes <literal>FOR EACH ROW</literal> of the
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specified operation is called once for every row that the operation
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modifies. For example, a <command>DELETE</command> that affects 10
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rows will cause any <literal>ON DELETE</literal> triggers on the
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target relation to be called 10 separate times, once for each
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deleted tuple. In contrast, a trigger that executes <literal>FOR
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EACH STATEMENT</literal> of the specified operation only executes
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once for any given operation, regardless of how many rows it
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modifies (in particular, an operation that modifies zero rows will
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still result in the execution of any applicable <literal>FOR EACH
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STATEMENT</literal> triggers).
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</para>
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<para>
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If multiple triggers of the same kind are defined for the same event,
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they will be fired in alphabetical order by name.
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</para>
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<para>
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<command>SELECT</command> does not modify any rows so you can not
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create <command>SELECT</command> triggers. Rules and views are more
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appropriate in such cases.
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</para>
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<para>
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Refer to the chapters on SPI and Triggers in the &cite-programmer;
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for more information.
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</para>
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</refsect1>
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<refsect1 id="SQL-CREATETRIGGER-notes">
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<title>Notes</title>
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<para>
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To create a trigger on a table, the user must have the
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<literal>TRIGGER</literal> privilege on the table.
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</para>
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<para>
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In <productname>PostgreSQL</productname> versions before 7.3, it was
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necessary to declare trigger functions as returning the placeholder
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type <type>opaque</>, rather than <type>trigger</>. To support loading
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of old dump files, <command>CREATE TRIGGER</> will accept a function
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declared as returning <type>opaque</>, but it will issue a NOTICE and
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change the function's declared return type to <type>trigger</>.
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</para>
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<para>
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Refer to the <xref linkend="sql-droptrigger" endterm="sql-droptrigger-title"> command for
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information on how to remove triggers.
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</para>
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</refsect1>
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<refsect1 id="R1-SQL-CREATETRIGGER-2">
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<title>Examples</title>
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<para>
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Check if the specified distributor code exists in the distributors
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table before appending or updating a row in the table films:
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<programlisting>
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CREATE TRIGGER if_dist_exists
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BEFORE INSERT OR UPDATE ON films FOR EACH ROW
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EXECUTE PROCEDURE check_primary_key ('did', 'distributors', 'did');
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</programlisting>
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</para>
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<para>
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Before cancelling a distributor or updating its code, remove every
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reference to the table films:
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<programlisting>
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CREATE TRIGGER if_film_exists
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BEFORE DELETE OR UPDATE ON distributors FOR EACH ROW
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EXECUTE PROCEDURE check_foreign_key (1, 'CASCADE', 'did', 'films', 'did');
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</programlisting>
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</para>
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<para>
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The second example can also be done by using a foreign key,
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constraint as in:
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<programlisting>
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CREATE TABLE distributors (
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did DECIMAL(3),
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name VARCHAR(40),
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CONSTRAINT if_film_exists
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FOREIGN KEY(did) REFERENCES films
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ON UPDATE CASCADE ON DELETE CASCADE
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);
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</programlisting>
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</para>
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</refsect1>
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<refsect1 id="SQL-CREATETRIGGER-compatibility">
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<title>Compatibility</title>
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<variablelist>
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<varlistentry>
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<term>SQL92</term>
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<listitem>
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<para>
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There is no <command>CREATE TRIGGER</command> statement in <acronym>SQL92</acronym>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>SQL99</term>
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<listitem>
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<para>
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The <command>CREATE TRIGGER</command> statement in
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<productname>PostgreSQL</productname> implements a subset of the
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SQL99 standard. The following functionality is missing:
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<itemizedlist>
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<listitem>
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<para>
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SQL99 allows triggers to fire on updates to specific columns
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(e.g., <literal>AFTER UPDATE OF col1, col2</literal>).
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</para>
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</listitem>
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<listitem>
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<para>
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SQL99 allows you to define aliases for the <quote>old</quote>
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and <quote>new</quote> rows or tables for use in the definition
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of the triggered action (e.g., <literal>CREATE TRIGGER ... ON
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tablename REFERENCING OLD ROW AS somename NEW ROW AS
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othername ...</literal>). Since
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<productname>PostgreSQL</productname> allows trigger
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procedures to be written in any number of user-defined
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languages, access to the data is handled in a
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language-specific way.
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</para>
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</listitem>
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<listitem>
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<para>
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<productname>PostgreSQL</productname> only allows the
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execution of a stored procedure for the triggered action.
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SQL99 allows the execution of a number of other SQL commands,
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such as <command>CREATE TABLE</command> as triggered action.
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This limitation is not hard to work around by creating a
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stored procedure that executes these commands.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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SQL99 specifies that multiple triggers should be fired in
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time-of-creation order. <productname>PostgreSQL</productname>
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uses name order, which was judged more convenient to work with.
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</para>
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<para>
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The ability to specify multiple actions for a single trigger
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using <literal>OR</literal> is a <productname>PostgreSQL</>
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extension of the SQL standard.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-createfunction" endterm="sql-createfunction-title"></member>
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<member><xref linkend="sql-altertrigger" endterm="sql-altertrigger-title"></member>
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<member><xref linkend="sql-droptrigger" endterm="sql-droptrigger-title"></member>
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<member>&cite-programmer;</member>
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</simplelist>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"../reference.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:"/usr/lib/sgml/catalog"
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sgml-local-ecat-files:nil
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End:
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-->
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