37.3. 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 to aTriggerData 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 castfcinfo->context to typeTriggerData * and make use of the pointed-toTriggerData structure. The function mustnot alter theTriggerData structure or any of the data it points to.
struct TriggerData is defined incommands/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:
typeAlways
T_TriggerData.tg_eventDescribes 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
INSERTcommand.TRIGGER_FIRED_BY_UPDATE(tg_event)Returns true if the trigger was fired by an
UPDATEcommand.TRIGGER_FIRED_BY_DELETE(tg_event)Returns true if the trigger was fired by a
DELETEcommand.TRIGGER_FIRED_BY_TRUNCATE(tg_event)Returns true if the trigger was fired by a
TRUNCATEcommand.
tg_relationA pointer to a structure describing the relation that the trigger fired for. Look at
utils/rel.hfor details about this structure. The most interesting things aretg_relation->rd_att(descriptor of the relation tuples) andtg_relation->rd_rel->relname(relation name; the type is notchar*butNameData; useSPI_getrelname(tg_relation)to get achar*if you need a copy of the name).tg_trigtupleA 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
INSERTorDELETEthen 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 ofINSERT) or skip the operation. For triggers on foreign tables, values of system columns herein are unspecified.tg_newtupleA pointer to the new version of the row, if the trigger was fired for an
UPDATE, andNULLif it is for anINSERTor aDELETE. This is what you have to return from the function if the event is anUPDATEand 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_triggerA pointer to a structure of type
Trigger, defined inutils/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
tgnameis the trigger's name,tgnargsis the number of arguments intgargs, andtgargsis an array of pointers to the arguments specified in theCREATE TRIGGERstatement. The other members are for internal use only.tg_trigtuplebufThe buffer containing
tg_trigtuple, orInvalidBufferif there is no such tuple or it is not stored in a disk buffer.tg_newtuplebufThe buffer containing
tg_newtuple, orInvalidBufferif there is no such tuple or it is not stored in a disk buffer.
A trigger function must return either aHeapTuple pointer or aNULL pointer (not an SQL null value, that is, do not setisNull true). Be careful to return eithertg_trigtuple ortg_newtuple, as appropriate, if you don't want to modify the row being operated on.