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$PostgreSQL: pgsql/doc/src/sgml/trigger.sgml,v 1.36 2004/08/13 16:17:19 tgl Exp $
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 <chapter id="triggers">
  <title>Triggers</title>

  <indexterm zone="triggers">
   <primary>trigger</primary>
  </indexterm>

  <para>
   This chapter describes how to write trigger functions.  Trigger
   functions can be written in C or in some of the available procedural
   languages.  It is not currently possible to write a SQL-language
   trigger function.
  </para>

  <sect1 id="trigger-definition">
   <title>Overview of Trigger Behavior</title>

   <para>
    A trigger can be defined to execute before or after an
    <command>INSERT</command>, <command>UPDATE</command>, or
    <command>DELETE</command> operation, either once per modified row,
    or once per <acronym>SQL</acronym> statement.
    If a trigger event occurs, the trigger's function is called
    at the appropriate time to handle the event.
   </para>

   <para>
    The trigger function must be defined before the trigger itself can be
    created.  The trigger function must be declared as a 
    function taking no arguments and returning type <literal>trigger</>.
    (The trigger function receives its input through a specially-passed
    <structname>TriggerData</> structure, not in the form of ordinary function
    arguments.)
   </para>

   <para>
    Once a suitable trigger function has been created, the trigger is
    established with
    <xref linkend="sql-createtrigger" endterm="sql-createtrigger-title">.
    The same trigger function can be used for multiple triggers.
   </para>

   <para>
    There are two types of triggers: per-row triggers and
    per-statement triggers. In a per-row trigger, the trigger function
    is invoked once for every row that is affected by the statement
    that fired the trigger. In contrast, a per-statement trigger is
    invoked only once when an appropriate statement is executed,
    regardless of the number of rows affected by that statement. In
    particular, a statement that affects zero rows will still result
    in the execution of any applicable per-statement triggers. These
    two types of triggers are sometimes called <quote>row-level
    triggers</quote> and <quote>statement-level triggers</quote>,
    respectively.
   </para>

   <para>
    Trigger functions invoked by per-statement triggers should always
    return <symbol>NULL</symbol>. Trigger functions invoked by per-row
    triggers can return a table row (a value of
    type <structname>HeapTuple</structname>) to the calling executor,
    if they choose.  A row-level trigger fired before an operation has
    the following choices:

    <itemizedlist>
     <listitem>
      <para>
       It can return <symbol>NULL</> to skip the operation for the
       current row. This instructs the executor to not perform the
       row-level operation that invoked the trigger (the insertion or
       modification of a particular table row).
      </para>
     </listitem>

     <listitem>
      <para>
       For row-level <command>INSERT</command>
       and <command>UPDATE</command> triggers only, the returned row
       becomes the row that will be inserted or will replace the row
       being updated.  This allows the trigger function to modify the
       row being inserted or updated.
      </para>
     </listitem>
    </itemizedlist>

    A row-level before trigger that does not intend to cause either of
    these behaviors must be careful to return as its result the same
    row that was passed in (that is, the <varname>NEW</varname> row
    for <command>INSERT</command> and <command>UPDATE</command>
    triggers, the <varname>OLD</varname> row for
    <command>DELETE</command> triggers).
   </para>

   <para>
    The return value is ignored for row-level triggers fired after an
    operation, and so they may as well return <symbol>NULL</>.
   </para>

   <para>
    If more than one trigger is defined for the same event on the same
    relation, the triggers will be fired in alphabetical order by
    trigger name.  In the case of before triggers, the
    possibly-modified row returned by each trigger becomes the input
    to the next trigger.  If any before trigger returns
    <symbol>NULL</>, the operation is abandoned and subsequent
    triggers are not fired.
   </para>

   <para>
    If a trigger function executes SQL commands then these
    commands may fire triggers again. This is known as cascading
    triggers.  There is no direct limitation on the number of cascade
    levels.  It is possible for cascades to cause a recursive invocation
    of the same trigger; for example, an <command>INSERT</command>
    trigger might execute a command that inserts an additional row
    into the same table, causing the <command>INSERT</command> trigger
    to be fired again.  It is the trigger programmer's responsibility
    to avoid infinite recursion in such scenarios.
   </para>

   <para>
    When a trigger is being defined, arguments can be specified for
    it.<indexterm><primary>trigger</><secondary>arguments for trigger
    functions</></indexterm> The purpose of including arguments in the
    trigger definition is to allow different triggers with similar
    requirements to call the same function.  As an example, there
    could be a generalized trigger function that takes as its
    arguments two column names and puts the current user in one and
    the current time stamp in the other.  Properly written, this
    trigger function would be independent of the specific table it is
    triggering on.  So the same function could be used for
    <command>INSERT</command> events on any table with suitable
    columns, to automatically track creation of records in a
    transaction table for example. It could also be used to track
    last-update events if defined as an <command>UPDATE</command>
    trigger.
   </para>

  </sect1>

  <sect1 id="trigger-datachanges">
   <title>Visibility of Data Changes</title>

   <para>
    If you execute SQL commands in your trigger function, and these
    commands access the table that the trigger is for, then
    you need to be aware of the data visibility rules, because they determine
    whether these SQL commands will see the data change that the trigger
    is fired for.  Briefly:

    <itemizedlist>

     <listitem>
      <para>
       Statement-level triggers follow simple visibility rules: none of
       the changes made by a statement are visible to statement-level
       triggers that are invoked before the statement, whereas all
       modifications are visible to statement-level after triggers.
      </para>
     </listitem>

     <listitem>
      <para>
       The data change (insertion, update, or deletion) causing the
       trigger to fire is naturally <emphasis>not</emphasis> visible
       to SQL commands executed in a row-level before trigger, because
       it hasn't happened yet.
      </para>
     </listitem>

     <listitem>
      <para>
       However, SQL commands executed in a row-level before
       trigger <emphasis>will</emphasis> see the effects of data
       changes for rows previously processed in the same outer
       command.  This requires caution, since the ordering of these
       change events is not in general predictable; a SQL command that
       affects multiple rows may visit the rows in any order.
      </para>
     </listitem>

     <listitem>
      <para>
       When a row-level after trigger is fired, all data changes made
       by the outer command are already complete, and are visible to
       the invoked trigger function.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    Further information about data visibility rules can be found in
    <xref linkend="spi-visibility">.  The example in <xref
    linkend="trigger-example"> contains a demonstration of these rules.
   </para>
  </sect1>

  <sect1 id="trigger-interface">
   <title>Writing Trigger Functions in C</title>

   <indexterm zone="trigger-interface">
    <primary>trigger</primary>
    <secondary>in C</secondary>
   </indexterm>

   <para>
    This section describes the low-level details of the interface to a
    trigger function.  This information is only needed when writing a
    trigger function in C.  If you are using a higher-level
    language then these details are handled for you.  The documentation
    of each procedural language explains how to write a trigger in that
    language.
   </para>

   <para>
    Trigger functions must use the <quote>version 1</> function manager
    interface.
   </para>

   <para>
    When a function is called by the trigger manager, it is not passed
    any normal arguments, but it is passed a <quote>context</>
    pointer pointing to a <structname>TriggerData</> structure.  C
    functions can check whether they were called from the trigger
    manager or not by executing the macro
<programlisting>
CALLED_AS_TRIGGER(fcinfo)
</programlisting>
    which expands to
<programlisting>
((fcinfo)->context != NULL && IsA((fcinfo)->context, TriggerData))
</programlisting>
    If this returns true, then it is safe to cast
    <literal>fcinfo->context</> to type <literal>TriggerData
    *</literal> and make use of the pointed-to
    <structname>TriggerData</> structure.  The function must
    <emphasis>not</emphasis> alter the <structname>TriggerData</>
    structure or any of the data it points to.
   </para>

   <para>
    <structname>struct TriggerData</structname> is defined in
    <filename>commands/trigger.h</filename>:

<programlisting>
typedef struct TriggerData
{
    NodeTag       type;
    TriggerEvent  tg_event;
    Relation      tg_relation;
    HeapTuple     tg_trigtuple;
    HeapTuple     tg_newtuple;
    Trigger      *tg_trigger;
} TriggerData;
</programlisting>

    where the members are defined as follows:

    <variablelist>
     <varlistentry>
      <term><structfield>type</></term>
      <listitem>
       <para>
        Always <literal>T_TriggerData</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_event</></term>
      <listitem>
       <para>
	Describes the event for which the function is called. You may use the
	following macros to examine <literal>tg_event</literal>:

	<variablelist>
	 <varlistentry>
	  <term><literal>TRIGGER_FIRED_BEFORE(tg_event)</literal></term>
	  <listitem>
	   <para>
	    Returns true if the trigger fired before the operation.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>TRIGGER_FIRED_AFTER(tg_event)</literal></term>
	  <listitem>
	   <para>
	    Returns true if the trigger fired after the operation.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>TRIGGER_FIRED_FOR_ROW(tg_event)</literal></term>
	  <listitem>
	   <para>
	    Returns true if the trigger fired for a row-level event.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>TRIGGER_FIRED_FOR_STATEMENT(tg_event)</literal></term>
	  <listitem>
	   <para>
	    Returns true if the trigger fired for a statement-level event.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>TRIGGER_FIRED_BY_INSERT(tg_event)</literal></term>
	  <listitem>
	   <para>
	    Returns true if the trigger was fired by an <command>INSERT</command> command.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>TRIGGER_FIRED_BY_UPDATE(tg_event)</literal></term>
	  <listitem>
	   <para>
	    Returns true if the trigger was fired by an <command>UPDATE</command> command.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term><literal>TRIGGER_FIRED_BY_DELETE(tg_event)</literal></term>
	  <listitem>
	   <para>
	    Returns true if the trigger was fired by a <command>DELETE</command> command.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_relation</></term>
      <listitem>
       <para>
	A pointer to a structure describing the relation that the trigger fired for.
	Look at <filename>utils/rel.h</> for details about
	this structure.  The most interesting things are
	<literal>tg_relation->rd_att</> (descriptor of the relation
	tuples) and <literal>tg_relation->rd_rel->relname</>
	(relation name; the type is not <type>char*</> but
	<type>NameData</>; use
	<literal>SPI_getrelname(tg_relation)</> to get a <type>char*</> if you
	need a copy of the name).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_trigtuple</></term>
      <listitem>
       <para>
	A pointer to the row for which the trigger was fired. This is
	the row being inserted, updated, or deleted.  If this trigger
	was fired for an <command>INSERT</command> or
	<command>DELETE</command> then this is what you should return
	to from the function if you don't want to replace the row with
	a different one (in the case of <command>INSERT</command>) or
	skip the operation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_newtuple</></term>
      <listitem>
       <para>
	A pointer to the new version of the row, if the trigger was
	fired for an <command>UPDATE</command>, and <symbol>NULL</> if
	it is for an <command>INSERT</command> or a
	<command>DELETE</command>. This is what you have to return
	from the function if the event is an <command>UPDATE</command>
	and you don't want to replace this row by a different one or
	skip the operation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><structfield>tg_trigger</></term>
      <listitem>
       <para>
	A pointer to a structure of type <structname>Trigger</>,
	defined in <filename>utils/rel.h</>:

<programlisting>
typedef struct Trigger
{
    Oid         tgoid;
    char       *tgname;
    Oid         tgfoid;
    int16       tgtype;
    bool        tgenabled;
    bool        tgisconstraint;
    Oid         tgconstrrelid;
    bool        tgdeferrable;
    bool        tginitdeferred;
    int16       tgnargs;
    int16       tgattr[FUNC_MAX_ARGS];
    char      **tgargs;
} Trigger;
</programlisting>

       where <structfield>tgname</> is the trigger's name,
       <structfield>tgnargs</> is number of arguments in
       <structfield>tgargs</>, and <structfield>tgargs</> is an array of
       pointers to the arguments specified in the <command>CREATE
       TRIGGER</command> statement. The other members are for internal use
       only.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    A trigger function must return either a
    <structname>HeapTuple</> pointer or a <symbol>NULL</> pointer
    (<emphasis>not</> a SQL NULL, that is, do not set isNull true).
    Be careful to return either
    <structfield>tg_trigtuple</> or <structfield>tg_newtuple</>,
    as appropriate, if you don't want to modify the row being operated on.
   </para>
  </sect1>

  <sect1 id="trigger-example">
   <title>A Complete Example</title>

   <para>
    Here is a very simple example of a trigger function written in C.
    (Examples of triggers written in procedural languages may be found
    in the documentation of the procedural languages.)
   </para>

   <para>
    The function <function>trigf</> reports the number of rows in the
    table <structname>ttest</> and skips the actual operation if the
    command attempts to insert a null value into the column
    <structfield>x</>. (So the trigger acts as a not-null constraint but
    doesn't abort the transaction.)
   </para>

   <para>
    First, the table definition:
<programlisting>
CREATE TABLE ttest (
    x integer
);
</programlisting>
   </para>

   <para>
    This is the source code of the trigger function:
<programlisting>
#include "postgres.h"
#include "executor/spi.h"       /* this is what you need to work with SPI */
#include "commands/trigger.h"   /* ... and triggers */

extern Datum trigf(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(trigf);

Datum
trigf(PG_FUNCTION_ARGS)
{
    TriggerData *trigdata = (TriggerData *) fcinfo->context;
    TupleDesc   tupdesc;
    HeapTuple   rettuple;
    char       *when;
    bool        checknull = false;
    bool        isnull;
    int         ret, i;

    /* make sure it's called as a trigger at all */
    if (!CALLED_AS_TRIGGER(fcinfo))
        elog(ERROR, "trigf: not called by trigger manager");

    /* tuple to return to executor */
    if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
        rettuple = trigdata->tg_newtuple;
    else
        rettuple = trigdata->tg_trigtuple;

    /* check for null values */
    if (!TRIGGER_FIRED_BY_DELETE(trigdata->tg_event)
        && TRIGGER_FIRED_BEFORE(trigdata->tg_event))
        checknull = true;

    if (TRIGGER_FIRED_BEFORE(trigdata->tg_event))
        when = "before";
    else
        when = "after ";

    tupdesc = trigdata->tg_relation->rd_att;

    /* connect to SPI manager */
    if ((ret = SPI_connect()) < 0)
        elog(INFO, "trigf (fired %s): SPI_connect returned %d", when, ret);

    /* get number of rows in table */
    ret = SPI_exec("SELECT count(*) FROM ttest", 0);

    if (ret < 0)
        elog(NOTICE, "trigf (fired %s): SPI_exec returned %d", when, ret);

    /* count(*) returns int8, so be careful to convert */
    i = DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
                                    SPI_tuptable->tupdesc,
                                    1,
                                    &amp;isnull));

    elog (INFO, "trigf (fired %s): there are %d rows in ttest", when, i);

    SPI_finish();

    if (checknull)
    {
        SPI_getbinval(rettuple, tupdesc, 1, &amp;isnull);
        if (isnull)
            rettuple = NULL;
    }

    return PointerGetDatum(rettuple);
}
</programlisting>
   </para>

   <para>
    After you have compiled the source code, declare the function and
    the triggers:
<programlisting>
CREATE FUNCTION trigf() RETURNS trigger
    AS '<replaceable>filename</>'
    LANGUAGE C;

CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest 
    FOR EACH ROW EXECUTE PROCEDURE trigf();

CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest 
    FOR EACH ROW EXECUTE PROCEDURE trigf();
</programlisting>
   </para>

   <para>
    Now you can test the operation of the trigger:
<screen>
=> INSERT INTO ttest VALUES (NULL);
INFO:  trigf (fired before): there are 0 rows in ttest
INSERT 0 0

-- Insertion skipped and AFTER trigger is not fired

=> SELECT * FROM ttest;
 x
---
(0 rows)

=> INSERT INTO ttest VALUES (1);
INFO:  trigf (fired before): there are 0 rows in ttest
INFO:  trigf (fired after ): there are 1 rows in ttest
                                       ^^^^^^^^
                             remember what we said about visibility.
INSERT 167793 1
vac=> SELECT * FROM ttest;
 x
---
 1
(1 row)

=> INSERT INTO ttest SELECT x * 2 FROM ttest;
INFO:  trigf (fired before): there are 1 rows in ttest
INFO:  trigf (fired after ): there are 2 rows in ttest
                                       ^^^^^^
                             remember what we said about visibility.
INSERT 167794 1
=> SELECT * FROM ttest;
 x
---
 1
 2
(2 rows)

=> UPDATE ttest SET x = NULL WHERE x = 2;
INFO:  trigf (fired before): there are 2 rows in ttest
UPDATE 0
=> UPDATE ttest SET x = 4 WHERE x = 2;
INFO:  trigf (fired before): there are 2 rows in ttest
INFO:  trigf (fired after ): there are 2 rows in ttest
UPDATE 1
vac=> SELECT * FROM ttest;
 x
---
 1
 4
(2 rows)

=> DELETE FROM ttest;
INFO:  trigf (fired before): there are 2 rows in ttest
INFO:  trigf (fired after ): there are 1 rows in ttest
INFO:  trigf (fired before): there are 1 rows in ttest
INFO:  trigf (fired after ): there are 0 rows in ttest
                                       ^^^^^^
                             remember what we said about visibility.
DELETE 2
=> SELECT * FROM ttest;
 x
---
(0 rows)
</screen>

   </para>

   <para>
    There are more complex examples in
    <filename>src/test/regress/regress.c</filename> and
    in <filename>contrib/spi</filename>.
   </para>
  </sect1>
 </chapter>

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