postgres/doc/src/sgml/ref/create_trigger.sgml

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$PostgreSQL: pgsql/doc/src/sgml/ref/create_trigger.sgml,v 1.49 2008/11/14 10:22:46 petere Exp $
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<refentry id="SQL-CREATETRIGGER">
 <refmeta>
  <refentrytitle id="SQL-CREATETRIGGER-TITLE">CREATE TRIGGER</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE TRIGGER</refname>
  <refpurpose>define a new trigger</refpurpose>
 </refnamediv>

 <indexterm zone="sql-createtrigger">
  <primary>CREATE TRIGGER</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
CREATE TRIGGER <replaceable class="PARAMETER">name</replaceable> { BEFORE | AFTER } { <replaceable class="PARAMETER">event</replaceable> [ OR ... ] }
    ON <replaceable class="PARAMETER">table</replaceable> [ FOR [ EACH ] { ROW | STATEMENT } ]
    EXECUTE PROCEDURE <replaceable class="PARAMETER">funcname</replaceable> ( <replaceable class="PARAMETER">arguments</replaceable> )
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE TRIGGER</command> creates a new trigger.  The
   trigger will be associated with the specified table and will
   execute the specified function <replaceable
   class="parameter">funcname</replaceable> when certain events occur.
  </para>

  <para>
   The trigger can be specified to fire either before the
   operation is attempted on a row (before constraints are checked and
   the <command>INSERT</command>, <command>UPDATE</command>, or
   <command>DELETE</command> is attempted) or after the operation has
   completed (after constraints are checked and the
   <command>INSERT</command>, <command>UPDATE</command>, or
   <command>DELETE</command> has completed). If the trigger fires
   before the event, the trigger can skip the operation for the
   current row, or change the row being inserted (for
   <command>INSERT</command> and <command>UPDATE</command> operations
   only). If the trigger fires after the event, all changes, including
   the last insertion, update, or deletion, are <quote>visible</quote>
   to the trigger.
  </para>

  <para>
   A trigger that is marked <literal>FOR EACH ROW</literal> is called
   once for every row that the operation modifies. For example, a
   <command>DELETE</command> that affects 10 rows will cause any
   <literal>ON DELETE</literal> triggers on the target relation to be
   called 10 separate times, once for each deleted row. In contrast, a
   trigger that is marked <literal>FOR EACH STATEMENT</literal> only
   executes once for any given operation, regardless of how many rows
   it modifies (in particular, an operation that modifies zero rows
   will still result in the execution of any applicable <literal>FOR
   EACH STATEMENT</literal> triggers).
  </para>

  <para>
   In addition, triggers may be defined to fire for a
   <command>TRUNCATE</command>, though only
   <literal>FOR EACH STATEMENT</literal>.
  </para>

  <para>
   If multiple triggers of the same kind are defined for the same event,
   they will be fired in alphabetical order by name.
  </para>

  <para>
   <command>SELECT</command> does not modify any rows so you cannot
   create <command>SELECT</command> triggers. Rules and views are more
   appropriate in such cases.
  </para>

  <para>
   Refer to <xref linkend="triggers"> for more information about triggers.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      The name to give the new trigger.  This must be distinct from
      the name of any other trigger for the same table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>BEFORE</literal></term>
    <term><literal>AFTER</literal></term>
    <listitem>
     <para>
      Determines whether the function is called before or after the
      event.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">event</replaceable></term>
    <listitem>
     <para>
      One of <command>INSERT</command>, <command>UPDATE</command>,
      <command>DELETE</command>, or <command>TRUNCATE</command>;
      this specifies the event that will fire the trigger. Multiple
      events can be specified using <literal>OR</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">table</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the table the trigger
      is for.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>FOR EACH ROW</literal></term>
    <term><literal>FOR EACH STATEMENT</literal></term>

    <listitem>
     <para>
      This specifies whether the trigger procedure should be fired
      once for every row affected by the trigger event, or just once
      per SQL statement. If neither is specified, <literal>FOR EACH
      STATEMENT</literal> is the default.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">funcname</replaceable></term>
    <listitem>
     <para>
      A user-supplied function that is declared as taking no arguments
      and returning type <literal>trigger</>, which is executed when
      the trigger fires.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">arguments</replaceable></term>
    <listitem>
     <para>
      An optional comma-separated list of arguments to be provided to
      the function when the trigger is executed.  The arguments are
      literal string constants.  Simple names and numeric constants
      can be written here, too, but they will all be converted to
      strings.  Please check the description of the implementation
      language of the trigger function about how the trigger arguments
      are accessible within the function; it might be different from
      normal function arguments.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1 id="SQL-CREATETRIGGER-notes">
  <title>Notes</title>

  <para>
   To create a trigger on a table, the user must have the
   <literal>TRIGGER</literal> privilege on the table.
  </para>

  <para>
   Use <xref linkend="sql-droptrigger"
   endterm="sql-droptrigger-title"> to remove a trigger.
  </para>

  <para>
   In <productname>PostgreSQL</productname> versions before 7.3, it was
   necessary to declare trigger functions as returning the placeholder
   type <type>opaque</>, rather than <type>trigger</>.  To support loading
   of old dump files, <command>CREATE TRIGGER</> will accept a function
   declared as returning <type>opaque</>, but it will issue a notice and
   change the function's declared return type to <type>trigger</>.
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-CREATETRIGGER-2">
  <title>Examples</title>

  <para>
   <xref linkend="trigger-example"> contains a complete example.
  </para>
 </refsect1>

 <refsect1 id="SQL-CREATETRIGGER-compatibility">
  <title>Compatibility</title>

  <para>
   The <command>CREATE TRIGGER</command> statement in
   <productname>PostgreSQL</productname> implements a subset of the
   <acronym>SQL</> standard. The following functionality is currently missing:

   <itemizedlist>
    <listitem>
     <para>
      SQL allows triggers to fire on updates to specific columns
      (e.g., <literal>AFTER UPDATE OF col1, col2</literal>).
     </para>
    </listitem>

    <listitem>
     <para>
      SQL allows you to define aliases for the <quote>old</quote>
      and <quote>new</quote> rows or tables for use in the definition
      of the triggered action (e.g., <literal>CREATE TRIGGER ... ON
      tablename REFERENCING OLD ROW AS somename NEW ROW AS othername
      ...</literal>).  Since <productname>PostgreSQL</productname>
      allows trigger procedures to be written in any number of
      user-defined languages, access to the data is handled in a
      language-specific way.
     </para>
    </listitem>

    <listitem>
     <para>
      <productname>PostgreSQL</productname> only allows the execution
      of a user-defined function for the triggered action.  The standard
      allows the execution of a number of other SQL commands, such as
      <command>CREATE TABLE</command> as the triggered action.  This
      limitation is not hard to work around by creating a user-defined
      function that executes the desired commands.
     </para>
    </listitem>

   </itemizedlist>
  </para>

  <para>
   SQL specifies that multiple triggers should be fired in
   time-of-creation order.  <productname>PostgreSQL</productname> uses
   name order, which was judged to be more convenient.
  </para>

  <para>
   SQL specifies that <literal>BEFORE DELETE</literal> triggers on cascaded
   deletes fire <emphasis>after</> the cascaded <literal>DELETE</> completes.
   The <productname>PostgreSQL</productname> behavior is for <literal>BEFORE
   DELETE</literal> to always fire before the delete action, even a cascading
   one.  This is considered more consistent.  There is also unpredictable
   behavior when <literal>BEFORE</literal> triggers modify rows that are later
   to be modified by referential actions.  This can lead to constraint violations
   or stored data that does not honor the referential constraint.
  </para>

  <para>
   The ability to specify multiple actions for a single trigger using
   <literal>OR</literal> is a <productname>PostgreSQL</> extension of
   the SQL standard.
  </para>

  <para>
   The ability to fire triggers for <command>TRUNCATE</command> is a
   <productname>PostgreSQL</> extension of the SQL standard.
  </para>

 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-createfunction" endterm="sql-createfunction-title"></member>
   <member><xref linkend="sql-altertrigger" endterm="sql-altertrigger-title"></member>
   <member><xref linkend="sql-droptrigger" endterm="sql-droptrigger-title"></member>
  </simplelist>
 </refsect1>
</refentry>