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 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;
TupleTableSlot *tg_trigslot;
TupleTableSlot *tg_newslot;
Tuplestorestate *tg_oldtable;
Tuplestorestate *tg_newtable;
const Bitmapset *tg_updatedcols;
} TriggerData;
where the members are defined as follows:
type
Always T_TriggerData.
tg_event
Describes the event for which the function is called. You can use the
following macros to examine 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
A pointer to a structure describing the relation that the trigger fired for.
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 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
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 INSERT or
DELETE 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 INSERT) or
skip the operation. For triggers on foreign tables, values of system
columns herein are unspecified.
tg_newtuple
A pointer 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 on foreign tables, values of system
columns herein are unspecified.
tg_trigger
A pointer to a structure of type Trigger,
defined in utils/reltrigger.h:
typedef struct Trigger
{
Oid tgoid;
char *tgname;
Oid tgfoid;
int16 tgtype;
char tgenabled;
bool tgisinternal;
bool tgisclone;
Oid tgconstrrelid;
Oid tgconstrindid;
Oid tgconstraint;
bool tgdeferrable;
bool tginitdeferred;
int16 tgnargs;
int16 tgnattr;
int16 *tgattr;
char **tgargs;
char *tgqual;
char *tgoldtable;
char *tgnewtable;
} 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_trigslot
The slot containing tg_trigtuple,
or a NULL pointer if there is no such tuple.
tg_newslot
The slot containing tg_newtuple,
or a NULL pointer if there is no such tuple.
tg_oldtable
A pointer to a structure of type Tuplestorestate
containing zero or more rows in the format specified by
tg_relation, or a NULL pointer
if there is no OLD TABLE transition relation.
tg_newtable
A pointer to a structure of type Tuplestorestate
containing zero or more rows in the format specified by
tg_relation, or a NULL pointer
if there is no NEW TABLE transition relation.
tg_updatedcols
For UPDATE triggers, a bitmap set indicating the
columns that were updated by the triggering command. Generic trigger
functions can use this to optimize actions by not having to deal with
columns that were not changed.
As an example, to determine whether a column with attribute number
attnum (1-based) is a member of this bitmap set,
call bms_is_member(attnum -
FirstLowInvalidHeapAttributeNumber,
trigdata->tg_updatedcols)).
For triggers other than UPDATE triggers, this will
be NULL.
To allow queries issued through SPI to reference transition tables, see SPI_register_trigger_data.
A trigger function must 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.