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<!-- $PostgreSQL: pgsql/doc/src/sgml/trigger.sgml,v 1.58 2009/08/04 22:04:37 petere Exp $ -->
<chapter id="triggers">
<title>Triggers</title>
<indexterm zone="triggers">
<primary>trigger</primary>
</indexterm>
<para>
This chapter provides general information about writing trigger functions.
Trigger functions can be written in most of the available procedural
languages, including
<application>PL/pgSQL</application> (<xref linkend="plpgsql">),
<application>PL/Tcl</application> (<xref linkend="pltcl">),
<application>PL/Perl</application> (<xref linkend="plperl">), and
<application>PL/Python</application> (<xref linkend="plpython">).
After reading this chapter, you should consult the chapter for
your favorite procedural language to find out the language-specific
details of writing a trigger in it.
</para>
<para>
It is also possible to write a trigger function in C, although
most people find it easier to use one of the procedural languages.
It is not currently possible to write a trigger function in the
plain SQL function language.
</para>
<sect1 id="trigger-definition">
<title>Overview of Trigger Behavior</title>
<para>
A trigger is a specification that the database should automatically
execute a particular function whenever a certain type of operation is
performed. Triggers can be defined to execute either before or after any
<command>INSERT</command>, <command>UPDATE</command>, or
<command>DELETE</command> operation, either once per modified row,
or once per <acronym>SQL</acronym> statement. Triggers can also fire
for <command>TRUNCATE</command> statements. 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>
<productname>PostgreSQL</productname> offers both <firstterm>per-row</>
triggers and <firstterm>per-statement</> triggers. With a per-row
trigger, the trigger function
is invoked once for each 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 <firstterm>row-level</>
triggers and <firstterm>statement-level</> triggers,
respectively. Triggers on <command>TRUNCATE</command> may only be
defined at statement-level.
</para>
<para>
Triggers are also classified as <firstterm>before</> triggers and
<firstterm>after</> triggers.
Statement-level before triggers naturally fire before the
statement starts to do anything, while statement-level after
triggers fire at the very end of the statement. Row-level before
triggers fire immediately before a particular row is operated on,
while row-level after triggers fire at the end of the statement
(but before any statement-level after triggers).
</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 can 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 for that row and subsequent
triggers are not fired.
</para>
<para>
Typically, row before triggers are used for checking or
modifying the data that will be inserted or updated. For example,
a before trigger might be used to insert the current time into a
<type>timestamp</type> column, or to check that two elements of the row are
consistent. Row after triggers are most sensibly
used to propagate the updates to other tables, or make consistency
checks against other tables. The reason for this division of labor is
that an after trigger can be certain it is seeing the final value of the
row, while a before trigger cannot; there might be other before triggers
firing after it. If you have no specific reason to make a trigger before
or after, the before case is more efficient, since the information about
the operation doesn't have to be saved until end of statement.
</para>
<para>
If a trigger function executes SQL commands then these
commands might 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>
<para>
Each programming language that supports triggers has its own method
for making the trigger input data available to the trigger function.
This input data includes the type of trigger event (e.g.,
<command>INSERT</command> or <command>UPDATE</command>) as well as any
arguments that were listed in <command>CREATE TRIGGER</>.
For a row-level trigger, the input data also includes the
<varname>NEW</varname> row for <command>INSERT</command> and
<command>UPDATE</command> triggers, and/or the <varname>OLD</varname> row
for <command>UPDATE</command> and <command>DELETE</command> triggers.
Statement-level triggers do not currently have any way to examine the
individual row(s) modified by the statement.
</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 can 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>
If your trigger function is written in any of the standard procedural
languages, then the above statements apply only if the function is
declared <literal>VOLATILE</>. Functions that are declared
<literal>STABLE</> or <literal>IMMUTABLE</> will not see changes made by
the calling command in any case.
</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
trigger functions in C. If you are using a higher-level language then
these details are handled for you. In most cases you should consider
using a procedural language before writing your triggers in C. 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)-&gt;context != NULL &amp;&amp; IsA((fcinfo)-&gt;context, TriggerData))
</programlisting>
If this returns true, then it is safe to cast
<literal>fcinfo-&gt;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;
Buffer tg_trigtuplebuf;
Buffer tg_newtuplebuf;
} 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 can 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>
<varlistentry>
<term><literal>TRIGGER_FIRED_BY_TRUNCATE(tg_event)</literal></term>
<listitem>
<para>
Returns true if the trigger was fired by a <command>TRUNCATE</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-&gt;rd_att</> (descriptor of the relation
tuples) and <literal>tg_relation-&gt;rd_rel-&gt;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
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;
Oid tgconstrindid;
Oid tgconstraint;
bool tgdeferrable;
bool tginitdeferred;
int16 tgnargs;
int16 tgnattr;
int16 *tgattr;
char **tgargs;
} Trigger;
</programlisting>
where <structfield>tgname</> is the trigger's name,
<structfield>tgnargs</> is the 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>
<varlistentry>
<term><structfield>tg_trigtuplebuf</></term>
<listitem>
<para>
The buffer containing <structfield>tg_trigtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
is no such tuple or it is not stored in a disk buffer.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><structfield>tg_newtuplebuf</></term>
<listitem>
<para>
The buffer containing <structfield>tg_newtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
is no such tuple or it is not stored in a disk buffer.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
A trigger function must return either a
<structname>HeapTuple</> pointer or a <symbol>NULL</> pointer
(<emphasis>not</> an SQL null value, that is, do not set <parameter>isNull</parameter> 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 Trigger Example</title>
<para>
Here is a very simple example of a trigger function written in C.
(Examples of triggers written in procedural languages can 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><![CDATA[
#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(ERROR, "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(ERROR, "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,
&isnull));
elog (INFO, "trigf (fired %s): there are %d rows in ttest", when, i);
SPI_finish();
if (checknull)
{
SPI_getbinval(rettuple, tupdesc, 1, &isnull);
if (isnull)
rettuple = NULL;
}
return PointerGetDatum(rettuple);
}
]]>
</programlisting>
</para>
<para>
After you have compiled the source code (see <xref
linkend="dfunc">), 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>
=&gt; 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
=&gt; SELECT * FROM ttest;
x
---
(0 rows)
=&gt; 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=&gt; SELECT * FROM ttest;
x
---
1
(1 row)
=&gt; 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
=&gt; SELECT * FROM ttest;
x
---
1
2
(2 rows)
=&gt; UPDATE ttest SET x = NULL WHERE x = 2;
INFO: trigf (fired before): there are 2 rows in ttest
UPDATE 0
=&gt; 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=&gt; SELECT * FROM ttest;
x
---
1
4
(2 rows)
=&gt; DELETE FROM ttest;
INFO: trigf (fired before): there are 2 rows in ttest
INFO: trigf (fired before): there are 1 rows in ttest
INFO: trigf (fired after ): there are 0 rows in ttest
INFO: trigf (fired after ): there are 0 rows in ttest
^^^^^^
remember what we said about visibility.
DELETE 2
=&gt; 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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