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; } 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; 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.
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.
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