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doc/src/sgml/ref/create_trigger.sgml
PostgreSQL documentation
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<refentry id="SQL-CREATETRIGGER">
 <refmeta>
  <refentrytitle>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 } ]
    [ WHEN ( <replaceable class="parameter">condition</replaceable> ) ]
    EXECUTE PROCEDURE <replaceable class="PARAMETER">function_name</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">function_name</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 effects of other triggers, 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>
   Also, a trigger definition can specify a Boolean <literal>WHEN</>
   condition, which will be tested to see whether the trigger should
   be fired.  In row-level triggers the <literal>WHEN</> condition can
   examine the old and/or new values of columns of the row.  Statement-level
   triggers can also have <literal>WHEN</> conditions, although the feature
   is not so useful for them since the condition cannot refer to any values
   in the table.
  </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>

     <para>
      For <command>UPDATE</command> triggers, it is possible to
      specify a list of columns using this syntax:
<synopsis>
UPDATE OF <replaceable>column_name1</replaceable> [, <replaceable>column_name2</replaceable> ... ]
</synopsis>
      The trigger will only fire if at least one of the listed columns
      is mentioned as a target of the update.
     </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">condition</replaceable></term>
    <listitem>
     <para>
      A Boolean expression that determines whether the trigger function
      will actually be executed.  If <literal>WHEN</> is specified, the
      function will only be called if the <replaceable
      class="parameter">condition</replaceable> returns <literal>true</>.
      In <literal>FOR EACH ROW</literal> triggers, the <literal>WHEN</>
      condition can refer to columns of the old and/or new row values
      by writing <literal>OLD.<replaceable
      class="parameter">column_name</replaceable></literal> or
      <literal>NEW.<replaceable
      class="parameter">column_name</replaceable></literal> respectively.
      Of course, <literal>INSERT</> triggers cannot refer to <literal>OLD</>
      and <literal>DELETE</> triggers cannot refer to <literal>NEW</>.
     </para>

     <para>
      Currently, <literal>WHEN</literal> expressions cannot contain
      subqueries.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">function_name</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 to find out how these arguments
      can be accessed 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"> to remove a trigger.
  </para>

  <para>
   A column-specific trigger (<literal>FOR UPDATE OF
   <replaceable>column_name</replaceable></literal>) will fire when any
   of its columns are listed as targets in the <command>UPDATE</>
   command's <literal>SET</> list.  It is possible for a column's value
   to change even when the trigger is not fired, because changes made to the
   row's contents by <literal>BEFORE UPDATE</> triggers are not considered.
   Conversely, a command such as <literal>UPDATE ... SET x = x ...</>
   will fire a trigger on column <literal>x</>, even though the column's
   value did not change.
  </para>

  <para>
   In a <literal>BEFORE</> trigger, the <literal>WHEN</> condition is
   evaluated just before the function is or would be executed, so using
   <literal>WHEN</> is not materially different from testing the same
   condition at the beginning of the trigger function.  Note in particular
   that the <literal>NEW</> row seen by the condition is the current value,
   as possibly modified by earlier triggers.  Also, a <literal>BEFORE</>
   trigger's <literal>WHEN</> condition is not allowed to examine the
   system columns of the <literal>NEW</> row (such as <literal>oid</>),
   because those won't have been set yet.
  </para>

  <para>
   In an <literal>AFTER</> trigger, the <literal>WHEN</> condition is
   evaluated just after the row update occurs, and it determines whether an
   event is queued to fire the trigger at the end of statement.  So when an
   <literal>AFTER</> trigger's <literal>WHEN</> condition does not return
   true, it is not necessary to queue an event nor to re-fetch the row at end
   of statement.  This can result in significant speedups in statements that
   modify many rows, if the trigger only needs to be fired for a few of the
   rows.
  </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="SQL-CREATETRIGGER-examples">
  <title>Examples</title>

  <para>
   Execute the function <function>check_account_update</> whenever
   a row of the table <literal>accounts</> is about to be updated:

<programlisting>
CREATE TRIGGER check_update
    BEFORE UPDATE ON accounts
    FOR EACH ROW
    EXECUTE PROCEDURE check_account_update();
</programlisting>

   The same, but only execute the function if column <literal>balance</>
   is specified as a target in the <command>UPDATE</> command:

<programlisting>
CREATE TRIGGER check_update
    BEFORE UPDATE OF balance ON accounts
    FOR EACH ROW
    EXECUTE PROCEDURE check_account_update();
</programlisting>

   This form only executes the function if column <literal>balance</>
   has in fact changed value:

<programlisting>
CREATE TRIGGER check_update
    BEFORE UPDATE ON accounts
    FOR EACH ROW
    WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
    EXECUTE PROCEDURE check_account_update();
</programlisting>

   Call a function to log updates of <literal>accounts</>, but only if
   something changed:

<programlisting>
CREATE TRIGGER log_update
    AFTER UPDATE ON accounts
    FOR EACH ROW
    WHEN (OLD.* IS DISTINCT FROM NEW.*)
    EXECUTE PROCEDURE log_account_update();
</programlisting>
  </para>

  <para>
   <xref linkend="trigger-example"> contains a complete example of a trigger
   function written in C.
  </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 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 or prevent
   updates during an update that is caused by a referential action.  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"></member>
   <member><xref linkend="sql-altertrigger"></member>
   <member><xref linkend="sql-droptrigger"></member>
  </simplelist>
 </refsect1>
</refentry>
