Triggers
PostgreSQL has various server-side
function interfaces. Server-side functions can be written in SQL,
C, or any defined procedural language. Trigger functions can be
written in C and most procedural languages, but not in SQL. Note that
statement-level trigger events are not supported in the current
version. You can currently specify BEFORE or AFTER on INSERT,
DELETE or UPDATE of a tuple as a trigger event.
Trigger Definition
If a trigger event occurs, the trigger manager (called by the Executor)
sets up a TriggerData information structure (described below) and calls
the trigger function to handle the event.
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 trigger.
(The trigger function receives its input through a TriggerData
structure, not in the form of ordinary function arguments.)
If the function is written in C, it must use the version 1
function manager interface.
The syntax for creating triggers is:
CREATE TRIGGER trigger [ BEFORE | AFTER ] [ INSERT | DELETE | UPDATE [ OR ... ] ]
ON relation FOR EACH [ ROW | STATEMENT ]
EXECUTE PROCEDURE procedure
(args);
where the arguments are:
trigger
The trigger must have a name distinct from all other triggers on
the same table. The name is needed
if you ever have to delete the trigger.
BEFORE
AFTER
Determines whether the function is called before or after
the event.
INSERT
DELETE
UPDATE
The next element of the command determines what event(s) will trigger
the function. Multiple events can be specified separated by OR.
relation
The relation name indicates which table the event applies to.
ROW
STATEMENT
The FOR EACH clause determines whether the trigger is fired for each
affected row or before (or after) the entire statement has completed.
Currently only the ROW case is supported.
procedure
The procedure name is the function to be called.
args
The arguments passed to the function in the TriggerData structure.
This is either empty or a list of one or more simple literal
constants (which will be passed to the function as strings).
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 field 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 INSERT events on any table with suitable fields,
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 UPDATE trigger.
Trigger functions return a HeapTuple to the calling executor. The return
value is ignored for triggers fired AFTER an operation,
but it allows BEFORE triggers to:
Return a NULL pointer to skip the operation for the
current tuple (and so the tuple will not be
inserted/updated/deleted).
For INSERT and UPDATE triggers only, the returned tuple becomes the
tuple which will be inserted or will replace the tuple being updated.
This allows the trigger function to modify the row being inserted or
updated.
A BEFORE trigger that does not intend to cause either of these behaviors
must be careful to return the same NEW tuple it is passed.
Note that there is no initialization performed by the CREATE TRIGGER
handler. This may be changed in the future.
If more than one trigger is defined for the same event on the same
relation, the triggers will be fired in alphabetical order by
name. In the case of BEFORE triggers, the possibly-modified tuple
returned by each trigger becomes the input to the next trigger.
If any BEFORE trigger returns NULL, the operation is
abandoned and subsequent triggers are not fired.
If a trigger function executes SQL-queries (using SPI) then these queries
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 recursive invocation of the same trigger --- for
example, an INSERT trigger might execute a query that inserts an
additional tuple into the same table, causing the INSERT trigger to be
fired again. It is the trigger programmer's
responsibility to avoid infinite recursion in such scenarios.
Interaction with the Trigger Manager
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 function
language then these details are handled for you.
The interface described here applies for
PostgreSQL 7.1 and later.
Earlier versions passed the TriggerData pointer in a global
variable CurrentTriggerData.
When a function is called by the trigger manager, it is not passed any
normal parameters, but it is passed a context pointer pointing to a
TriggerData structure. C functions can check 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;
} TriggerData;
where the members are defined as follows:
type
Always T_TriggerData if this is a trigger event.
tg_event
describes the event for which the function is called. You may use the
following macros to examine tg_event:
TRIGGER_FIRED_BEFORE(tg_event)
returns TRUE if trigger fired BEFORE.
TRIGGER_FIRED_AFTER(tg_event)
Returns TRUE if trigger fired AFTER.
TRIGGER_FIRED_FOR_ROW(event)
Returns TRUE if trigger fired for
a ROW-level event.
TRIGGER_FIRED_FOR_STATEMENT(event)
Returns TRUE if trigger fired for
STATEMENT-level event.
TRIGGER_FIRED_BY_INSERT(event)
Returns TRUE if trigger fired by INSERT.
TRIGGER_FIRED_BY_DELETE(event)
Returns TRUE if trigger fired by DELETE.
TRIGGER_FIRED_BY_UPDATE(event)
Returns TRUE if trigger fired by UPDATE.
tg_relation
is a pointer to structure describing the triggered
relation. Look at 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's name. This is not char*, but
NameData. Use
SPI_getrelname(tg_relation) to get char* if you
need a copy of the name).
tg_trigtuple
is a pointer to the tuple for which the trigger is fired. This is the tuple
being inserted (if INSERT), deleted (if DELETE) or updated (if UPDATE).
If INSERT/DELETE then this is what you are to return to Executor if
you don't want to replace tuple with another one (INSERT) or skip the
operation.
tg_newtuple
is a pointer to the new version of tuple if UPDATE and NULL if this is
for an INSERT or a DELETE. This is what you are to return to Executor if
UPDATE and you don't want to replace this tuple with another one or skip
the operation.
tg_trigger
is pointer to structure Trigger defined in utils/rel.h:
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;
where tgname is the trigger's name,
tgnargs is number of arguments in
tgargs, tgargs is an array of
pointers to the arguments specified in the CREATE TRIGGER
statement. Other members are for internal use only.
Visibility of Data Changes
PostgreSQL data changes visibility rule: during a query execution, data
changes made by the query itself (via SQL-function, SPI-function, triggers)
are invisible to the query scan. For example, in query
INSERT INTO a SELECT * FROM a;
tuples inserted are invisible for SELECT scan. In effect, this
duplicates the database table within itself (subject to unique index
rules, of course) without recursing.
But keep in mind this notice about visibility in the SPI documentation:
Changes made by query Q are visible by queries that are started after
query Q, no matter whether they are started inside Q (during the
execution of Q) or after Q is done.
This is true for triggers as well so, though a tuple being inserted
(tg_trigtuple) is not visible to queries in a BEFORE trigger, this tuple
(just inserted) is visible to queries in an AFTER trigger, and to queries
in BEFORE/AFTER triggers fired after this!
Examples
There are more complex examples in
src/test/regress/regress.c and
in contrib/spi.
Here is a very simple example of trigger usage. Function trigf reports
the number of tuples in the triggered relation ttest and skips the
operation if the query attempts to insert a null value into x (i.e - it acts as a
not-null constraint but doesn't abort the transaction).
#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 trigdata is pointing at what I expect */
if (!CALLED_AS_TRIGGER(fcinfo))
elog(ERROR, "trigf: not fired 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 tuples in relation */
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 as of PG 7.2, so be careful to convert */
i = (int) DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
SPI_tuptable->tupdesc,
1,
&isnull));
elog (NOTICE, "trigf (fired %s): there are %d tuples in ttest", when, i);
SPI_finish();
if (checknull)
{
(void) SPI_getbinval(rettuple, tupdesc, 1, &isnull);
if (isnull)
rettuple = NULL;
}
return PointerGetDatum(rettuple);
}
Now, compile and create the trigger function:
CREATE FUNCTION trigf () RETURNS TRIGGER AS
'...path_to_so' LANGUAGE C;
CREATE TABLE ttest (x int4);
vac=> CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest
FOR EACH ROW EXECUTE PROCEDURE trigf();
CREATE
vac=> CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest
FOR EACH ROW EXECUTE PROCEDURE trigf();
CREATE
vac=> INSERT INTO ttest VALUES (NULL);
WARNING: trigf (fired before): there are 0 tuples in ttest
INSERT 0 0
-- Insertion skipped and AFTER trigger is not fired
vac=> SELECT * FROM ttest;
x
---
(0 rows)
vac=> INSERT INTO ttest VALUES (1);
INFO: trigf (fired before): there are 0 tuples in ttest
INFO: trigf (fired after ): there are 1 tuples in ttest
^^^^^^^^
remember what we said about visibility.
INSERT 167793 1
vac=> SELECT * FROM ttest;
x
---
1
(1 row)
vac=> INSERT INTO ttest SELECT x * 2 FROM ttest;
INFO: trigf (fired before): there are 1 tuples in ttest
INFO: trigf (fired after ): there are 2 tuples in ttest
^^^^^^^^
remember what we said about visibility.
INSERT 167794 1
vac=> SELECT * FROM ttest;
x
---
1
2
(2 rows)
vac=> UPDATE ttest SET x = NULL WHERE x = 2;
INFO: trigf (fired before): there are 2 tuples in ttest
UPDATE 0
vac=> UPDATE ttest SET x = 4 WHERE x = 2;
INFO: trigf (fired before): there are 2 tuples in ttest
INFO: trigf (fired after ): there are 2 tuples in ttest
UPDATE 1
vac=> SELECT * FROM ttest;
x
---
1
4
(2 rows)
vac=> DELETE FROM ttest;
INFO: trigf (fired before): there are 2 tuples in ttest
INFO: trigf (fired after ): there are 1 tuples in ttest
INFO: trigf (fired before): there are 1 tuples in ttest
INFO: trigf (fired after ): there are 0 tuples in ttest
^^^^^^^^
remember what we said about visibility.
DELETE 2
vac=> SELECT * FROM ttest;
x
---
(0 rows)