trigger This chapter provides general information about writing trigger functions. Trigger functions can be written in most of the available procedural languages, including PL/pgSQL (), PL/Tcl (), PL/Perl (), and PL/Python (). After reading this chapter, you should consult the chapter for your favorite procedural language to find out the language-specific details of writing a trigger in it. It is also possible to write a trigger function in C, although most people find it easier to use one of the procedural languages. It is not currently possible to write a trigger function in the plain SQL function language. A trigger is a specification that the database should automatically execute a particular function whenever a certain type of operation is performed. Triggers can be defined to execute either before or after any INSERT, UPDATE, or DELETE operation, either once per modified row, or once per SQL statement. If a trigger event occurs, the trigger's function is called at the appropriate time 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 specially-passed TriggerData structure, not in the form of ordinary function arguments.) Once a suitable trigger function has been created, the trigger is established with . The same trigger function can be used for multiple triggers. PostgreSQL offers both per-row triggers and per-statement triggers. With a per-row trigger, the trigger function is invoked once for each row that is affected by the statement that fired the trigger. In contrast, a per-statement trigger is invoked only once when an appropriate statement is executed, regardless of the number of rows affected by that statement. In particular, a statement that affects zero rows will still result in the execution of any applicable per-statement triggers. These two types of triggers are sometimes called row-level triggers and statement-level triggers, respectively. Triggers are also classified as before triggers and after triggers. Statement-level before triggers naturally fire before the statement starts to do anything, while statement-level after triggers fire at the very end of the statement. Row-level before triggers fire immediately before a particular row is operated on, while row-level after triggers fire at the end of the statement (but before any statement-level after triggers). Trigger functions invoked by per-statement triggers should always return NULL. Trigger functions invoked by per-row triggers can return a table row (a value of type HeapTuple) to the calling executor, if they choose. A row-level trigger fired before an operation has the following choices: It can return NULL to skip the operation for the current row. This instructs the executor to not perform the row-level operation that invoked the trigger (the insertion or modification of a particular table row). For row-level INSERT and UPDATE triggers only, the returned row becomes the row that will be inserted or will replace the row being updated. This allows the trigger function to modify the row being inserted or updated. A row-level before trigger that does not intend to cause either of these behaviors must be careful to return as its result the same row that was passed in (that is, the NEW row for INSERT and UPDATE triggers, the OLD row for DELETE triggers). The return value is ignored for row-level triggers fired after an operation, and so they can return NULL. If more than one trigger is defined for the same event on the same relation, the triggers will be fired in alphabetical order by trigger name. In the case of before triggers, the possibly-modified row returned by each trigger becomes the input to the next trigger. If any before trigger returns NULL, the operation is abandoned for that row and subsequent triggers are not fired. Typically, row before triggers are used for checking or modifying the data that will be inserted or updated. For example, a before trigger might be used to insert the current time into a timestamp column, or to check that two elements of the row are consistent. Row after triggers are most sensibly used to propagate the updates to other tables, or make consistency checks against other tables. The reason for this division of labor is that an after trigger can be certain it is seeing the final value of the row, while a before trigger cannot; there might be other before triggers firing after it. If you have no specific reason to make a trigger before or after, the before case is more efficient, since the information about the operation doesn't have to be saved until end of statement. If a trigger function executes SQL commands then these commands might 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 a recursive invocation of the same trigger; for example, an INSERT trigger might execute a command that inserts an additional row 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. When a trigger is being defined, arguments can be specified for it.triggerarguments for trigger functions 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 column 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 columns, 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. Each programming language that supports triggers has its own method for making the trigger input data available to the trigger function. This input data includes the type of trigger event (e.g., INSERT or UPDATE) as well as any arguments that were listed in CREATE TRIGGER. For a row-level trigger, the input data also includes the NEW row for INSERT and UPDATE triggers, and/or the OLD row for UPDATE and DELETE triggers. Statement-level triggers do not currently have any way to examine the individual row(s) modified by the statement. If you execute SQL commands in your trigger function, and these commands access the table that the trigger is for, then you need to be aware of the data visibility rules, because they determine whether these SQL commands will see the data change that the trigger is fired for. Briefly: Statement-level triggers follow simple visibility rules: none of the changes made by a statement are visible to statement-level triggers that are invoked before the statement, whereas all modifications are visible to statement-level after triggers. The data change (insertion, update, or deletion) causing the trigger to fire is naturally not visible to SQL commands executed in a row-level before trigger, because it hasn't happened yet. However, SQL commands executed in a row-level before trigger will see the effects of data changes for rows previously processed in the same outer command. This requires caution, since the ordering of these change events is not in general predictable; a SQL command that affects multiple rows can visit the rows in any order. When a row-level after trigger is fired, all data changes made by the outer command are already complete, and are visible to the invoked trigger function. Further information about data visibility rules can be found in . The example in contains a demonstration of these rules. trigger 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 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; } 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_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. 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. 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. tg_trigger A pointer to a structure of type Trigger, defined in utils/rel.h: typedef struct Trigger { Oid tgoid; char *tgname; Oid tgfoid; int16 tgtype; bool tgenabled; bool tgisconstraint; Oid tgconstrrelid; Oid tgconstraint; bool tgdeferrable; bool tginitdeferred; int16 tgnargs; int16 tgnattr; int16 *tgattr; char **tgargs; } Trigger; where tgname is the trigger's name, tgnargs is 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_trigtuplebuf The buffer containing tg_trigtuple, or InvalidBuffer if there is no such tuple or it is not stored in a disk buffer. tg_newtuplebuf The buffer containing tg_newtuple, or InvalidBuffer if there is no such tuple or it is not stored in a disk buffer. 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. Here is a very simple example of a trigger function written in C. (Examples of triggers written in procedural languages can be found in the documentation of the procedural languages.) The function trigf reports the number of rows in the table ttest and skips the actual operation if the command attempts to insert a null value into the column x. (So the trigger acts as a not-null constraint but doesn't abort the transaction.) First, the table definition: CREATE TABLE ttest ( x integer ); This is the source code of the trigger function: #include "postgres.h" #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 it's called as a trigger at all */ if (!CALLED_AS_TRIGGER(fcinfo)) elog(ERROR, "trigf: not called 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 rows in table */ 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, so be careful to convert */ i = DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0], SPI_tuptable->tupdesc, 1, &isnull)); elog (INFO, "trigf (fired %s): there are %d rows in ttest", when, i); SPI_finish(); if (checknull) { SPI_getbinval(rettuple, tupdesc, 1, &isnull); if (isnull) rettuple = NULL; } return PointerGetDatum(rettuple); } After you have compiled the source code, declare the function and the triggers: CREATE FUNCTION trigf() RETURNS trigger AS 'filename' LANGUAGE C; CREATE TRIGGER tbefore BEFORE INSERT OR UPDATE OR DELETE ON ttest FOR EACH ROW EXECUTE PROCEDURE trigf(); CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest FOR EACH ROW EXECUTE PROCEDURE trigf(); Now you can test the operation of the trigger: => INSERT INTO ttest VALUES (NULL); INFO: trigf (fired before): there are 0 rows in ttest INSERT 0 0 -- Insertion skipped and AFTER trigger is not fired => SELECT * FROM ttest; x --- (0 rows) => INSERT INTO ttest VALUES (1); INFO: trigf (fired before): there are 0 rows in ttest INFO: trigf (fired after ): there are 1 rows in ttest ^^^^^^^^ remember what we said about visibility. INSERT 167793 1 vac=> SELECT * FROM ttest; x --- 1 (1 row) => INSERT INTO ttest SELECT x * 2 FROM ttest; INFO: trigf (fired before): there are 1 rows in ttest INFO: trigf (fired after ): there are 2 rows in ttest ^^^^^^ remember what we said about visibility. INSERT 167794 1 => SELECT * FROM ttest; x --- 1 2 (2 rows) => UPDATE ttest SET x = NULL WHERE x = 2; INFO: trigf (fired before): there are 2 rows in ttest UPDATE 0 => UPDATE ttest SET x = 4 WHERE x = 2; INFO: trigf (fired before): there are 2 rows in ttest INFO: trigf (fired after ): there are 2 rows in ttest UPDATE 1 vac=> SELECT * FROM ttest; x --- 1 4 (2 rows) => DELETE FROM ttest; INFO: trigf (fired before): there are 2 rows in ttest INFO: trigf (fired before): there are 1 rows in ttest INFO: trigf (fired after ): there are 0 rows in ttest INFO: trigf (fired after ): there are 0 rows in ttest ^^^^^^ remember what we said about visibility. DELETE 2 => SELECT * FROM ttest; x --- (0 rows) There are more complex examples in src/test/regress/regress.c and in contrib/spi.