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Chapter 36. Triggers
If your trigger function is written in any of the standard procedural languages, then the above state-
ments apply only if the function is declared
VOLATILE
. Functions that are declared
STABLE
or
IMMUTABLE
will not see changes made by the calling command in any case.
Further information about data visibility rules can be found in Section 44.4. The example in Section
36.4 contains a demonstration of these rules.
36.3. Writing Trigger Functions in C
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.
Trigger functions must use the “version 1” function manager interface.
When a function is called by the trigger manager, it is not passed any normal arguments, but it is
passed a “context” pointer pointing toa
TriggerData
structure. C functions cancheck whether they
were called from the trigger manager or not by executing the macro:
CALLED_AS_TRIGGER(fcinfo)
which expands to:
((fcinfo)->context != NULL && IsA((fcinfo)->context, TriggerData))
If this returns true, then it is safe to cast
fcinfo->context
to type
TriggerData
*
and make use
of the pointed-to
TriggerData
structure. The function must not alter the
TriggerData
structure or
any of the data it points to.
struct TriggerData
is defined in
commands/trigger.h
:
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;
where the members are defined as follows:
type
Always
T_TriggerData
.
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Chapter 36. Triggers
tg_event
Describes the event for which thefunctionis called.Youcanuse the followingmacrostoexamine
tg_event
:
TRIGGER_FIRED_BEFORE(tg_event)
Returns true if the trigger fired before the operation.
TRIGGER_FIRED_AFTER(tg_event)
Returns true if the trigger fired after the operation.
TRIGGER_FIRED_INSTEAD(tg_event)
Returns true if the trigger fired instead of the operation.
TRIGGER_FIRED_FOR_ROW(tg_event)
Returns true if the trigger fired for a row-level event.
TRIGGER_FIRED_FOR_STATEMENT(tg_event)
Returns true if the trigger fired for a statement-level event.
TRIGGER_FIRED_BY_INSERT(tg_event)
Returns true if the trigger was fired by an
INSERT
command.
TRIGGER_FIRED_BY_UPDATE(tg_event)
Returns true if the trigger was fired by an
UPDATE
command.
TRIGGER_FIRED_BY_DELETE(tg_event)
Returns true if the trigger was fired by a
DELETE
command.
TRIGGER_FIRED_BY_TRUNCATE(tg_event)
Returns true if the trigger was fired by a
TRUNCATE
command.
tg_relation
Apointer toa structure describing therelation thatthe trigger firedfor. Lookat
utils/rel.h
for
details about this structure. The most interesting things are
tg_relation->rd_att
(descriptor
of the relation tuples) and
tg_relation->rd_rel->relname
(relation name; the type is not
char
*
but
NameData
;use
SPI_getrelname(tg_relation)
to get a
char
*
if you need a
copy of the name).
tg_trigtuple
Apointer 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
INSERT
or
DELETE
then this is what you should re-
turn from the function if you don’t want to replace the row with a different one (in the case of
INSERT
)or skip the operation. For triggers on foreign tables, values of system columns herein
are unspecified.
tg_newtuple
Apointer to the new version of the row, if the trigger was fired for an
UPDATE
,and
NULL
if it
is for an
INSERT
or a
DELETE
.This is what you have to return from the function if the event is
an
UPDATE
and you don’t want to replace this row by a different one or skip the operation. For
triggers onforeign tables, values of system columns herein are unspecified.
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Chapter 36. Triggers
tg_trigger
Apointer toa structure of type
Trigger
,defined in
utils/reltrigger.h
:
typedef struct Trigger
{
Oid
tgoid;
char
*
tgname;
Oid
tgfoid;
int16
tgtype;
char
tgenabled;
bool
tgisinternal;
Oid
tgconstrrelid;
Oid
tgconstrindid;
Oid
tgconstraint;
bool
tgdeferrable;
bool
tginitdeferred;
int16
tgnargs;
int16
tgnattr;
int16
*
tgattr;
char
**
tgargs;
char
*
tgqual;
} Trigger;
where
tgname
is the trigger’s name,
tgnargs
is the number of arguments in
tgargs
, and
tgargs
is an array of pointers to the arguments specified in the
CREATE TRIGGER
statement.
The other members are for internal use only.
tg_trigtuplebuf
The buffer containing
tg_trigtuple
,or
InvalidBuffer
if there is no such tuple or it is not
stored in a disk buffer.
tg_newtuplebuf
The buffer containing
tg_newtuple
,or
InvalidBuffer
if there is no such tuple or it is not
stored in a disk buffer.
Atrigger functionmust return either a
HeapTuple
pointer or a
NULL
pointer (not an SQL null value,
that is, do not set
isNull
true). Be careful to return either
tg_trigtuple
or
tg_newtuple
,as
appropriate, if you don’t want to modify the row being operated on.
36.4. A Complete Trigger Example
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.)
The function
trigf
reports the number of rows in the table
ttest
and skips the actual operation
if the command attempts to insert a null value into the column
x
.(So the trigger acts as a not-null
constraint but doesn’t abort the transaction.)
First, the table definition:
CREATE TABLE ttest (
x integer
);
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Chapter 36. Triggers
This is the source code of the trigger function:
#include "postgres.h"
#include "executor/spi.h"
/
*
this is what you need to work with SPI
*
/
#include "commands/trigger.h"
/
*
... triggers ...
*
/
#include "utils/rel.h"
/
*
... and relations
*
/
#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif
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);
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Chapter 36. Triggers
/
*
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);
}
After you have compiled the source code (see Section 35.9.6), declare the function and the triggers:
CREATE FUNCTION trigf() RETURNS trigger
AS ’
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();
Now you can test the operation of the trigger:
=> 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
---
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Chapter 36. Triggers
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 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
=> SELECT
*
FROM ttest;
x
---
(0 rows)
There are more complex examples in
src/test/regress/regress.c
and in spi.
993
Chapter 37. Event Triggers
To supplement the trigger mechanism discussed in Chapter 36, PostgreSQL also provides event trig-
gers. Unlike regular triggers, which are attached toa single table andcapture onlyDML events, event
triggers are global to a particular database and are capable of capturing DDL events.
Like regular triggers, event triggers can be written in any procedural language that includes event
trigger support, or in C, but not in plain SQL.
37.1. Overview of Event Trigger Behavior
An event trigger fires whenever the eventwith whichit is associatedoccurs inthe database inwhich it
is defined. Currently, the only supported events are
ddl_command_start
,
ddl_command_end
and
sql_drop
.Support for additional events may be added in future releases.
The
ddl_command_start
event occurs just before the execution of a
CREATE
,
ALTER
,or
DROP
command. No check whether the affected object exists or doesn’t exist is performed before the event
trigger fires. As an exception, however, this event does not occur for DDL commands targetingshared
objects — databases, roles, and tablespaces — or for commands targeting event triggers themselves.
The event trigger mechanism does not support these object types.
ddl_command_start
also occurs
just before the execution of a
SELECT INTO
command, since this is equivalent to
CREATE TABLE
AS
.
The
ddl_command_end
event occurs just after the execution of this same set of commands.
The
sql_drop
event occurs just before the
ddl_command_end
event trigger for any operation that
drops database objects. To list the objects that have been dropped, use the set-returning function
pg_event_trigger_dropped_objects()
from the
sql_drop
event trigger code (see Section
9.28). Note that the trigger is executed after the objects have been deleted from the system catalogs,
so it’s not possible to look them up anymore.
Event triggers (like other functions) cannot be executed in an aborted transaction. Thus, if a DDL
command fails with an error, any associated
ddl_command_end
triggers will not be executed. Con-
versely, if a
ddl_command_start
trigger fails with an error, no further event triggers will fire, and
no attempt will be made to execute the command itself. Similarly, if a
ddl_command_end
trigger
fails with an error, the effects of the DDL statement will be rolled back, just as they would be in any
other case where the containing transaction aborts.
For a complete list of commands supported by the event trigger mechanism, see Section 37.2.
Event triggers are created using the command CREATE EVENT TRIGGER. In order to create an
event trigger, you must first create a function with the special return type
event_trigger
.This
function need not (and may not) return a value; the return type serves merely as a signal that the
function is to be invoked as an event trigger.
If more than one event trigger is defined for a particular event, they will fire in alphabetical order by
trigger name.
Atrigger definition can also specify a
WHEN
condition so that, for example, a
ddl_command_start
trigger can be fired only for particular commands which the user wishes to intercept. A common use
of such triggers is to restrict the range of DDL operations which users may perform.
994
Chapter 37. Event Triggers
37.2. Event Trigger Firing Matrix
Table 37-1 lists all commands for which event triggers are supported.
Table 37-1. Event Trigger Support by Command Tag
command tag
ddl_command_start
ddl_command_end
sql_drop
ALTER AGGREGATE
X
X
-
ALTER COLLATION
X
X
-
ALTER CONVERSION
X
X
-
ALTER DOMAIN
X
X
-
ALTER EXTENSION
X
X
-
ALTER FOREIGN
DATA WRAPPER
X
X
-
ALTER FOREIGN
TABLE
X
X
X
ALTER FUNCTION
X
X
-
ALTER LANGUAGE
X
X
-
ALTER OPERATOR
X
X
-
ALTER OPERATOR
CLASS
X
X
-
ALTER OPERATOR
FAMILY
X
X
-
ALTER SCHEMA
X
X
-
ALTER SEQUENCE
X
X
-
ALTER SERVER
X
X
-
ALTER TABLE
X
X
X
ALTER TEXT SEARCH
CONFIGURATION
X
X
-
ALTER TEXT SEARCH
DICTIONARY
X
X
-
ALTER TEXT SEARCH
PARSER
X
X
-
ALTER TEXT SEARCH
TEMPLATE
X
X
-
ALTER TRIGGER
X
X
-
ALTER TYPE
X
X
-
ALTER USER
MAPPING
X
X
-
ALTER VIEW
X
X
-
CREATE AGGREGATE
X
X
-
CREATE CAST
X
X
-
CREATE COLLATION
X
X
-
CREATE CONVERSION
X
X
-
CREATE DOMAIN
X
X
-
995
Chapter 37. Event Triggers
command tag
ddl_command_start
ddl_command_end
sql_drop
CREATE EXTENSION
X
X
-
CREATE FOREIGN
DATA WRAPPER
X
X
-
CREATE FOREIGN
TABLE
X
X
-
CREATE FUNCTION
X
X
-
CREATE INDEX
X
X
-
CREATE LANGUAGE
X
X
-
CREATE OPERATOR
X
X
-
CREATE OPERATOR
CLASS
X
X
-
CREATE OPERATOR
FAMILY
X
X
-
CREATE RULE
X
X
-
CREATE SCHEMA
X
X
-
CREATE SEQUENCE
X
X
-
CREATE SERVER
X
X
-
CREATE TABLE
X
X
-
CREATE TABLE AS
X
X
-
CREATE TEXT
SEARCH
CONFIGURATION
X
X
-
CREATE TEXT
SEARCH DICTIONARY
X
X
-
CREATE TEXT
SEARCH PARSER
X
X
-
CREATE TEXT
SEARCH TEMPLATE
X
X
-
CREATE TRIGGER
X
X
-
CREATE TYPE
X
X
-
CREATE USER
MAPPING
X
X
-
CREATE VIEW
X
X
-
DROP AGGREGATE
X
X
X
DROP CAST
X
X
X
DROP COLLATION
X
X
X
DROP CONVERSION
X
X
X
DROP DOMAIN
X
X
X
DROP EXTENSION
X
X
X
DROP FOREIGN DATA
WRAPPER
X
X
X
DROP FOREIGN
TABLE
X
X
X
DROP FUNCTION
X
X
X
996
Chapter 37. Event Triggers
command tag
ddl_command_start
ddl_command_end
sql_drop
DROP INDEX
X
X
X
DROP LANGUAGE
X
X
X
DROP OPERATOR
X
X
X
DROP OPERATOR
CLASS
X
X
X
DROP OPERATOR
FAMILY
X
X
X
DROP OWNED
X
X
X
DROP RULE
X
X
X
DROP SCHEMA
X
X
X
DROP SEQUENCE
X
X
X
DROP SERVER
X
X
X
DROP TABLE
X
X
X
DROP TEXT SEARCH
CONFIGURATION
X
X
X
DROP TEXT SEARCH
DICTIONARY
X
X
X
DROP TEXT SEARCH
PARSER
X
X
X
DROP TEXT SEARCH
TEMPLATE
X
X
X
DROP TRIGGER
X
X
X
DROP TYPE
X
X
X
DROP USER MAPPING
X
X
X
DROP VIEW
X
X
X
SELECT INTO
X
X
-
37.3. Writing Event Trigger Functions in C
This section describes the low-level details of the interface to an event trigger function. This informa-
tion is only neededwhen writingevent trigger functions in C. If you are using a higher-level language
thenthese details are handled for you. In most cases you shouldconsider usinga procedural language
before writing your event triggers in C. The documentation of each procedural language explains how
to write an event trigger inthat language.
Event trigger functions must use the “version 1” function manager interface.
When a function is called by the event trigger manager, it is not passed any normal arguments, but
it is passed a “context” pointer pointing to a
EventTriggerData
structure. C functions can check
whether they were called from the event trigger manager or not by executing the macro:
CALLED_AS_EVENT_TRIGGER(fcinfo)
which expands to:
((fcinfo)->context != NULL && IsA((fcinfo)->context, EventTriggerData))
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