path: root/doc/src/sgml/ref/create_trigger.sgml

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doc/src/sgml/ref/create_trigger.sgml
PostgreSQL documentation
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<refentry id="sql-createtrigger">
 <indexterm zone="sql-createtrigger">
  <primary>CREATE TRIGGER</primary>
 </indexterm>

 <indexterm>
  <primary>transition tables</primary>
  <seealso>ephemeral named relation</seealso>
 </indexterm>

 <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>

 <refsynopsisdiv>
<synopsis>
CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER <replaceable class="parameter">name</replaceable> { BEFORE | AFTER | INSTEAD OF } { <replaceable class="parameter">event</replaceable> [ OR ... ] }
    ON <replaceable class="parameter">table_name</replaceable>
    [ FROM <replaceable class="parameter">referenced_table_name</replaceable> ]
    [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
    [ REFERENCING { { OLD | NEW } TABLE [ AS ] <replaceable class="parameter">transition_relation_name</replaceable> } [ ... ] ]
    [ FOR [ EACH ] { ROW | STATEMENT } ]
    [ WHEN ( <replaceable class="parameter">condition</replaceable> ) ]
    EXECUTE { FUNCTION | PROCEDURE } <replaceable class="parameter">function_name</replaceable> ( <replaceable class="parameter">arguments</replaceable> )

<phrase>where <replaceable class="parameter">event</replaceable> can be one of:</phrase>

    INSERT
    UPDATE [ OF <replaceable class="parameter">column_name</replaceable> [, ... ] ]
    DELETE
    TRUNCATE
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE TRIGGER</command> creates a new trigger.
   <command>CREATE OR REPLACE TRIGGER</command> will either create a
   new trigger, or replace an existing trigger.  The
   trigger will be associated with the specified table, view, or foreign table
   and will execute the specified
   function <replaceable class="parameter">function_name</replaceable> when
   certain operations are performed on that table.
  </para>

  <para>
   To replace the current definition of an existing trigger, use
   <command>CREATE OR REPLACE TRIGGER</command>, specifying the existing
   trigger's name and parent table.  All other properties are replaced.
  </para>

  <para>
   The trigger can be specified to fire 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); or instead of the operation
   (in the case of inserts, updates or deletes on a view).
   If the trigger fires before or instead of 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>
   Triggers that are specified to fire <literal>INSTEAD OF</literal> the trigger
   event must be marked <literal>FOR EACH ROW</literal>, and can only be defined
   on views. <literal>BEFORE</literal> and <literal>AFTER</literal> triggers on a view
   must be marked as <literal>FOR EACH STATEMENT</literal>.
  </para>

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

  <para>
   The following table summarizes which types of triggers may be used on
   tables, views, and foreign tables:
  </para>

  <informaltable id="supported-trigger-types">
   <tgroup cols="4">
    <thead>
     <row>
      <entry>When</entry>
      <entry>Event</entry>
      <entry>Row-level</entry>
      <entry>Statement-level</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry align="center" morerows="1"><literal>BEFORE</literal></entry>
      <entry align="center"><command>INSERT</command>/<command>UPDATE</command>/<command>DELETE</command></entry>
      <entry align="center">Tables and foreign tables</entry>
      <entry align="center">Tables, views, and foreign tables</entry>
     </row>
     <row>
      <entry align="center"><command>TRUNCATE</command></entry>
      <entry align="center">&mdash;</entry>
      <entry align="center">Tables</entry>
     </row>
     <row>
      <entry align="center" morerows="1"><literal>AFTER</literal></entry>
      <entry align="center"><command>INSERT</command>/<command>UPDATE</command>/<command>DELETE</command></entry>
      <entry align="center">Tables and foreign tables</entry>
      <entry align="center">Tables, views, and foreign tables</entry>
     </row>
     <row>
      <entry align="center"><command>TRUNCATE</command></entry>
      <entry align="center">&mdash;</entry>
      <entry align="center">Tables</entry>
     </row>
     <row>
      <entry align="center" morerows="1"><literal>INSTEAD OF</literal></entry>
      <entry align="center"><command>INSERT</command>/<command>UPDATE</command>/<command>DELETE</command></entry>
      <entry align="center">Views</entry>
      <entry align="center">&mdash;</entry>
     </row>
     <row>
      <entry align="center"><command>TRUNCATE</command></entry>
      <entry align="center">&mdash;</entry>
      <entry align="center">&mdash;</entry>
     </row>
    </tbody>
   </tgroup>
  </informaltable>

  <para>
   Also, a trigger definition can specify a Boolean <literal>WHEN</literal>
   condition, which will be tested to see whether the trigger should
   be fired.  In row-level triggers the <literal>WHEN</literal> condition can
   examine the old and/or new values of columns of the row.  Statement-level
   triggers can also have <literal>WHEN</literal> 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>
   When the <literal>CONSTRAINT</literal> option is specified, this command creates a
   <firstterm>constraint trigger</firstterm>.<indexterm><primary>trigger</primary>
   <secondary>constraint trigger</secondary></indexterm>
   This is the same as a regular trigger
   except that the timing of the trigger firing can be adjusted using
   <link linkend="sql-set-constraints"><command>SET CONSTRAINTS</command></link>.
   Constraint triggers must be <literal>AFTER ROW</literal> triggers on plain
   tables (not foreign tables).  They
   can be fired either at the end of the statement causing the triggering
   event, or at the end of the containing transaction; in the latter case they
   are said to be <firstterm>deferred</firstterm>.  A pending deferred-trigger firing
   can also be forced to happen immediately by using <command>SET
   CONSTRAINTS</command>.  Constraint triggers are expected to raise an exception
   when the constraints they implement are violated.
  </para>

  <para>
   The <literal>REFERENCING</literal> option enables collection
   of <firstterm>transition relations</firstterm>, which are row sets that include all
   of the rows inserted, deleted, or modified by the current SQL statement.
   This feature lets the trigger see a global view of what the statement did,
   not just one row at a time.  This option is only allowed for
   an <literal>AFTER</literal> trigger that is not a constraint trigger; also, if
   the trigger is an <literal>UPDATE</literal> trigger, it must not specify
   a <replaceable class="parameter">column_name</replaceable> list.
   <literal>OLD TABLE</literal> may only be specified once, and only for a trigger
   that can fire on <literal>UPDATE</literal> or <literal>DELETE</literal>; it creates a
   transition relation containing the <firstterm>before-images</firstterm> of all rows
   updated or deleted by the statement.
   Similarly, <literal>NEW TABLE</literal> may only be specified once, and only for
   a trigger that can fire on <literal>UPDATE</literal> or <literal>INSERT</literal>;
   it creates a transition relation containing the <firstterm>after-images</firstterm>
   of all rows updated or inserted by the statement.
  </para>

  <para>
   <command>SELECT</command> does not modify any rows so you cannot
   create <command>SELECT</command> triggers.  Rules and views may provide
   workable solutions to problems that seem to need <command>SELECT</command>
   triggers.
  </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.
      The name cannot be schema-qualified &mdash; the trigger inherits the
      schema of its table.  For a constraint trigger, this is also the name to
      use when modifying the trigger's behavior using
      <command>SET CONSTRAINTS</command>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>BEFORE</literal></term>
    <term><literal>AFTER</literal></term>
    <term><literal>INSTEAD OF</literal></term>
    <listitem>
     <para>
      Determines whether the function is called before, after, or instead of
      the event.  A constraint trigger can only be specified as
      <literal>AFTER</literal>.
     </para>
    </listitem>
   </varlistentry>

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

     <para>
      For <literal>UPDATE</literal> events, 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 <command>UPDATE</command> command
      or if one of the listed columns is a generated column that depends on a
      column that is the target of the <command>UPDATE</command>.
     </para>

     <para>
      <literal>INSTEAD OF UPDATE</literal> events do not allow a list of columns.
      A column list cannot be specified when requesting transition relations,
      either.
     </para>
    </listitem>
   </varlistentry>

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

   <varlistentry>
    <term><replaceable class="parameter">referenced_table_name</replaceable></term>
    <listitem>
     <para>
      The (possibly schema-qualified) name of another table referenced by the
      constraint.  This option is used for foreign-key constraints and is not
      recommended for general use.  This can only be specified for
      constraint triggers.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEFERRABLE</literal></term>
    <term><literal>NOT DEFERRABLE</literal></term>
    <term><literal>INITIALLY IMMEDIATE</literal></term>
    <term><literal>INITIALLY DEFERRED</literal></term>
    <listitem>
     <para>
      The default timing of the trigger.
      See the <xref linkend="sql-createtable"/> documentation for details of
      these constraint options.  This can only be specified for constraint
      triggers.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>REFERENCING</literal></term>
    <listitem>
     <para>
      This keyword immediately precedes the declaration of one or two
      relation names that provide access to the transition relations of the
      triggering statement.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>OLD TABLE</literal></term>
    <term><literal>NEW TABLE</literal></term>
    <listitem>
     <para>
      This clause indicates whether the following relation name is for the
      before-image transition relation or the after-image transition
      relation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">transition_relation_name</replaceable></term>
    <listitem>
     <para>
      The (unqualified) name to be used within the trigger for this
      transition relation.
     </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 function 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.  Constraint triggers can only
      be specified <literal>FOR EACH ROW</literal>.
     </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</literal> is specified, the
      function will only be called if the <replaceable
      class="parameter">condition</replaceable> returns <literal>true</literal>.
      In <literal>FOR EACH ROW</literal> triggers, the <literal>WHEN</literal>
      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</literal> triggers cannot refer to <literal>OLD</literal>
      and <literal>DELETE</literal> triggers cannot refer to <literal>NEW</literal>.
     </para>

     <para><literal>INSTEAD OF</literal> triggers do not support <literal>WHEN</literal>
      conditions.
     </para>

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

     <para>
      Note that for constraint triggers, evaluation of the <literal>WHEN</literal>
      condition is not deferred, but occurs immediately after the row update
      operation is performed. If the condition does not evaluate to true then
      the trigger is not queued for deferred execution.
     </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</literal>, which is executed when
      the trigger fires.
     </para>

     <para>
      In the syntax of <literal>CREATE TRIGGER</literal>, the keywords
      <literal>FUNCTION</literal> and <literal>PROCEDURE</literal> are
      equivalent, but the referenced function must in any case be a function,
      not a procedure.  The use of the keyword <literal>PROCEDURE</literal>
      here is historical and deprecated.
     </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 or replace a trigger on a table, the user must have the
   <literal>TRIGGER</literal> privilege on the table.  The user must
   also have <literal>EXECUTE</literal> privilege on the trigger function.
  </para>

  <para>
   Use <link linkend="sql-droptrigger"><command>DROP TRIGGER</command></link> to remove a trigger.
  </para>

  <para>
   Creating a row-level trigger on a partitioned table will cause an
   identical <quote>clone</quote> trigger to be created on each of its
   existing partitions; and any partitions created or attached later will have
   an identical trigger, too.  If there is a conflictingly-named trigger on a
   child partition already, an error occurs unless <command>CREATE OR REPLACE
   TRIGGER</command> is used, in which case that trigger is replaced with a
   clone trigger.  When a partition is detached from its parent, its clone
   triggers are removed.
  </para>

  <para>
   A column-specific trigger (one defined using the <literal>UPDATE OF
   <replaceable>column_name</replaceable></literal> syntax) will fire when any
   of its columns are listed as targets in the <command>UPDATE</command>
   command's <literal>SET</literal> 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</literal> triggers are not considered.
   Conversely, a command such as <literal>UPDATE ... SET x = x ...</literal>
   will fire a trigger on column <literal>x</literal>, even though the column's
   value did not change.
  </para>

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

  <para>
   In an <literal>AFTER</literal> trigger, the <literal>WHEN</literal> 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</literal> trigger's <literal>WHEN</literal> 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 some cases it is possible for a single SQL command to fire more than
   one kind of trigger.  For instance an <command>INSERT</command> with
   an <literal>ON CONFLICT DO UPDATE</literal> clause may cause both insert and
   update operations, so it will fire both kinds of triggers as needed.
   The transition relations supplied to triggers are
   specific to their event type; thus an <command>INSERT</command> trigger
   will see only the inserted rows, while an <command>UPDATE</command>
   trigger will see only the updated rows.
  </para>

  <para>
   Row updates or deletions caused by foreign-key enforcement actions, such
   as <literal>ON UPDATE CASCADE</literal> or <literal>ON DELETE SET NULL</literal>, are
   treated as part of the SQL command that caused them (note that such
   actions are never deferred).  Relevant triggers on the affected table will
   be fired, so that this provides another way in which an SQL command might
   fire triggers not directly matching its type.  In simple cases, triggers
   that request transition relations will see all changes caused in their
   table by a single original SQL command as a single transition relation.
   However, there are cases in which the presence of an <literal>AFTER ROW</literal>
   trigger that requests transition relations will cause the foreign-key
   enforcement actions triggered by a single SQL command to be split into
   multiple steps, each with its own transition relation(s).  In such cases,
   any statement-level triggers that are present will be fired once per
   creation of a transition relation set, ensuring that the triggers see
   each affected row in a transition relation once and only once.
  </para>

  <para>
   Statement-level triggers on a view are fired only if the action on the
   view is handled by a row-level <literal>INSTEAD OF</literal> trigger.
   If the action is handled by an <literal>INSTEAD</literal> rule, then
   whatever statements are emitted by the rule are executed in place of the
   original statement naming the view, so that the triggers that will be
   fired are those on tables named in the replacement statements.
   Similarly, if the view is automatically updatable, then the action is
   handled by automatically rewriting the statement into an action on the
   view's base table, so that the base table's statement-level triggers are
   the ones that are fired.
  </para>

  <para>
   Modifying a partitioned table or a table with inheritance children fires
   statement-level triggers attached to the explicitly named table, but not
   statement-level triggers for its partitions or child tables.  In contrast,
   row-level triggers are fired on the rows in affected partitions or
   child tables, even if they are not explicitly named in the query.
   If a statement-level trigger has been defined with transition relations
   named by a <literal>REFERENCING</literal> clause, then before and after
   images of rows are visible from all affected partitions or child tables.
   In the case of inheritance children, the row images include only columns
   that are present in the table that the trigger is attached to.
  </para>

  <para>
   Currently, row-level triggers with transition relations cannot be defined
   on partitions or inheritance child tables.  Also, triggers on partitioned
   tables may not be <literal>INSTEAD OF</literal>.
  </para>

  <para>
   Currently, the <literal>OR REPLACE</literal> option is not supported for
   constraint triggers.
  </para>

  <para>
   Replacing an existing trigger within a transaction that has already
   performed updating actions on the trigger's table is not recommended.
   Trigger firing decisions, or portions of firing decisions, that have
   already been made will not be reconsidered, so the effects could be
   surprising.
  </para>

  <para>
   There are a few built-in trigger functions that can be used to
   solve common problems without having to write your own trigger code;
   see <xref linkend="functions-trigger"/>.
  </para>
 </refsect1>

 <refsect1 id="sql-createtrigger-examples">
  <title>Examples</title>

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

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

   Modify that trigger definition to only execute the function if
   column <literal>balance</literal> is specified as a target in
   the <command>UPDATE</command> command:

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

   This form only executes the function if column <literal>balance</literal>
   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 FUNCTION check_account_update();
</programlisting>

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

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

   Execute the function <function>view_insert_row</function> for each row to insert
   rows into the tables underlying a view:

<programlisting>
CREATE TRIGGER view_insert
    INSTEAD OF INSERT ON my_view
    FOR EACH ROW
    EXECUTE FUNCTION view_insert_row();
</programlisting>

   Execute the function <function>check_transfer_balances_to_zero</function> for each
   statement to confirm that the <literal>transfer</literal> rows offset to a net of
   zero:

<programlisting>
CREATE TRIGGER transfer_insert
    AFTER INSERT ON transfer
    REFERENCING NEW TABLE AS inserted
    FOR EACH STATEMENT
    EXECUTE FUNCTION check_transfer_balances_to_zero();
</programlisting>

   Execute the function <function>check_matching_pairs</function> for each row to
   confirm that changes are made to matching pairs at the same time (by the
   same statement):

<programlisting>
CREATE TRIGGER paired_items_update
    AFTER UPDATE ON paired_items
    REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
    FOR EACH ROW
    EXECUTE FUNCTION check_matching_pairs();
</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>

  <!--
   It's not clear whether SQL/MED contemplates triggers on foreign tables.
   Its <drop basic column definition> General Rules do mention the possibility
   of a reference from a trigger column list.  On the other hand, nothing
   overrides the fact that CREATE TRIGGER only targets base tables.  For now,
   do not document the compatibility status of triggers on foreign tables.
  -->

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

   <itemizedlist>
    <listitem>
     <para>
      While transition table names for <literal>AFTER</literal> triggers are
      specified using the <literal>REFERENCING</literal> clause in the standard way,
      the row variables used in <literal>FOR EACH ROW</literal> triggers may not be
      specified in a <literal>REFERENCING</literal> clause.  They are available in a
      manner that is dependent on the language in which the trigger function
      is written, but is fixed for any one language.  Some languages
      effectively behave as though there is a <literal>REFERENCING</literal> clause
      containing <literal>OLD ROW AS OLD NEW ROW AS NEW</literal>.
     </para>
    </listitem>

    <listitem>
     <para>
      The standard allows transition tables to be used with
      column-specific <literal>UPDATE</literal> triggers, but then the set of rows
      that should be visible in the transition tables depends on the
      trigger's column list.  This is not currently implemented by
      <productname>PostgreSQL</productname>.
     </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</emphasis> the cascaded <literal>DELETE</literal> 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 nonstandard
   behavior if <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</productname> extension of
   the SQL standard.
  </para>

  <para>
   The ability to fire triggers for <command>TRUNCATE</command> is a
   <productname>PostgreSQL</productname> extension of the SQL standard, as is the
   ability to define statement-level triggers on views.
  </para>

  <para>
   <command>CREATE CONSTRAINT TRIGGER</command> is a
   <productname>PostgreSQL</productname> extension of the <acronym>SQL</acronym>
   standard.
   So is the <literal>OR REPLACE</literal> option.
  </para>

 </refsect1>

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

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