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38.3. Writing Trigger Functions in C
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38.3. 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 “[.quote]#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 “[.quote]#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;
    Buffer           tg_trigtuplebuf;
    Buffer           tg_newtuplebuf;
    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.
[.term]#`+tg_trigtuplebuf+`#::
  The buffer containing `+tg_trigtuple+`, or `+InvalidBuffer+` if there is no such tuple or it is not stored in a disk buffer.
[.term]#`+tg_newtuplebuf+`#::
  The buffer containing `+tg_newtuple+`, or `+InvalidBuffer+` if there is no such tuple or it is not stored in a disk buffer.
[.term]#`+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.
[.term]#`+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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38.2. Visibility of Data Changes

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38.4. A Complete Trigger Example