7.2. Triggers
This chapter provides general information about writing trigger functions. Trigger functions can be written in most of the available procedural languages, including PL/pgSQL (plpgsql), PL/Tcl (pltcl), PL/Perl (plperl), and PL/Python (plpython). 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.
7.2.1. Overview of Trigger Behavior
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 attached to tables (partitioned or not), views, and foreign tables.
On tables and foreign tables, 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. UPDATE triggers can moreover be set to fire only if certain columns are mentioned in the SET clause of the UPDATE statement. Triggers can also fire for TRUNCATE statements. If a trigger event occurs, the trigger’s function is called at the appropriate time to handle the event.
On views, triggers can be defined to execute instead of INSERT, UPDATE, or DELETE operations. Such INSTEAD OF triggers are fired once for each row that needs to be modified in the view. It is the responsibility of the trigger’s function to perform the necessary modifications to the view’s underlying base table(s) and, where appropriate, return the modified row as it will appear in the view. Triggers on views can also be defined to execute once per SQL statement, before or after INSERT, UPDATE, or DELETE operations. However, such triggers are fired only if there is also an INSTEAD OF trigger on the view. Otherwise, any statement targeting the view must be rewritten into a statement affecting its underlying base table(s), and then the triggers that will be fired are the ones attached to the base table(s).
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 sql-createtrigger. 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 on TRUNCATE may only be defined at statement level, not per-row.
Triggers are also classified according to whether they fire before, after, or instead of the operation. These are referred to as BEFORE triggers, AFTER triggers, and INSTEAD OF triggers respectively. 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. These types of triggers may be defined on tables, views, or foreign tables. 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). These types of triggers may only be defined on tables and foreign tables, not views. INSTEAD OF triggers may only be defined on views, and only at row level; they fire immediately as each row in the view is identified as needing to be operated on.
The execution of an AFTER trigger can be deferred to the end of the transaction, rather than the end of the statement, if it was defined as a constraint trigger. In all cases, a trigger is executed as part of the same transaction as the statement that triggered it, so if either the statement or the trigger causes an error, the effects of both will be rolled back.
A statement that targets a parent table in an inheritance or partitioning hierarchy does not cause the statement-level triggers of affected child tables to be fired; only the parent table’s statement-level triggers are fired. However, row-level triggers of any affected child tables will be fired.
If an INSERT contains an ON CONFLICT DO UPDATE clause, it is possible that the effects of row-level BEFORE INSERT triggers and row-level BEFORE UPDATE triggers can both be applied in a way that is apparent from the final state of the updated row, if an EXCLUDED column is referenced. There need not be an EXCLUDED column reference for both sets of row-level BEFORE triggers to execute, though. The possibility of surprising outcomes should be considered when there are both BEFORE INSERT and BEFORE UPDATE row-level triggers that change a row being inserted/updated (this can be problematic even if the modifications are more or less equivalent, if they’re not also idempotent). Note that statement-level UPDATE triggers are executed when ON CONFLICT DO UPDATE is specified, regardless of whether or not any rows were affected by the UPDATE (and regardless of whether the alternative UPDATE path was ever taken). An INSERT with an ON CONFLICT DO UPDATE clause will execute statement-level BEFORE INSERT triggers first, then statement-level BEFORE UPDATE triggers, followed by statement-level AFTER UPDATE triggers and finally statement-level AFTER INSERT triggers.
If an UPDATE on a partitioned table causes a row to move to another partition, it will be performed as a DELETE from the original partition followed by an INSERT into the new partition. In this case, all row-level BEFORE UPDATE triggers and all row-level BEFORE DELETE triggers are fired on the original partition. Then all row-level BEFORE INSERT triggers are fired on the destination partition. The possibility of surprising outcomes should be considered when all these triggers affect the row being moved. As far as AFTER ROW triggers are concerned, AFTER DELETE and AFTER INSERT triggers are applied; but AFTER UPDATE triggers are not applied because the UPDATE has been converted to a DELETE and an INSERT. As far as statement-level triggers are concerned, none of the DELETE or INSERT triggers are fired, even if row movement occurs; only the UPDATE triggers defined on the target table used in the UPDATE statement will be fired.
No separate triggers are defined for MERGE. Instead, statement-level or row-level UPDATE, DELETE, and INSERT triggers are fired depending on (for statement-level triggers) what actions are specified in the MERGE query and (for row-level triggers) what actions are performed.
While running a MERGE command, statement-level BEFORE and AFTER triggers are fired for events specified in the actions of the MERGE command, irrespective of whether or not the action is ultimately performed. This is the same as an UPDATE statement that updates no rows, yet statement-level triggers are fired. The row-level triggers are fired only when a row is actually updated, inserted or deleted. So it’s perfectly legal that while statement-level triggers are fired for certain types of action, no row-level triggers are fired for the same kind of action.
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, modification, or deletion 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).
A row-level INSTEAD OF trigger should either return NULL to indicate that it did not modify any data from the view’s underlying base tables, or it should return the view row that was passed in (the NEW row for INSERT and UPDATE operations, or the OLD row for DELETE operations). A nonnull return value is used to signal that the trigger performed the necessary data modifications in the view. This will cause the count of the number of rows affected by the command to be incremented. For INSERT and UPDATE operations only, the trigger may modify the NEW row before returning it. This will change the data returned by INSERT RETURNING or UPDATE RETURNING, and is useful when the view will not show exactly the same data that was provided.
The return value is ignored for row-level triggers fired after an operation, and so they can return NULL.
Some considerations apply for generated columns. Stored generated columns are computed after BEFORE triggers and before AFTER triggers. Therefore, the generated value can be inspected in AFTER triggers. In BEFORE triggers, the OLD row contains the old generated value, as one would expect, but the NEW row does not yet contain the new generated value and should not be accessed. In the C language interface, the content of the column is undefined at this point; a higher-level programming language should prevent access to a stored generated column in the NEW row in a BEFORE trigger. Changes to the value of a generated column in a BEFORE trigger are ignored and will be overwritten.
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 and INSTEAD OF triggers, the possibly-modified row returned by each trigger becomes the input to the next trigger. If any BEFORE or INSTEAD OF trigger returns NULL, the operation is abandoned for that row and subsequent triggers are not fired (for that row).
A trigger definition can also specify a Boolean WHEN condition, which will be tested to see whether the trigger should be fired. In row-level triggers the WHEN condition can examine the old and/or new values of columns of the row. (Statement-level triggers can also have WHEN conditions, although the feature is not so useful for them.) In a BEFORE trigger, the WHEN condition is evaluated just before the function is or would be executed, so using WHEN is not materially different from testing the same condition at the beginning of the trigger function. However, in an AFTER trigger, the WHEN condition is evaluated just after the row update occurs, and it determines whether an event is queued to fire the trigger at the end of statement. So when an AFTER trigger’s WHEN condition does not return true, it is not necessary to queue an event nor to re-fetch the row at end of statement. This can result in significant speedups in statements that modify many rows, if the trigger only needs to be fired for a few of the rows. INSTEAD OF triggers do not support WHEN conditions.
Typically, row-level 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-level 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. 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.
By default, statement-level triggers do not have any way to examine the individual row(s) modified by the statement. But an AFTER STATEMENT trigger can request that transition tables be created to make the sets of affected rows available to the trigger. AFTER ROW triggers can also request transition tables, so that they can see the total changes in the table as well as the change in the individual row they are currently being fired for. The method for examining the transition tables again depends on the programming language that is being used, but the typical approach is to make the transition tables act like read-only temporary tables that can be accessed by SQL commands issued within the trigger function.
7.2.2. Visibility of Data Changes
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 BEFORE triggers, 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; an SQL command that affects multiple rows can visit the rows in any order.
Similarly, a row-level INSTEAD OF trigger will see the effects of data changes made by previous firings of INSTEAD OF triggers in the same outer command.
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.
If your trigger function is written in any of the standard procedural languages, then the above statements apply only if the function is declared VOLATILE. Functions that are declared STABLE or IMMUTABLE will not see changes made by the calling command in any case.
Further information about data visibility rules can be found in spi-visibility. The example in trigger-example contains a demonstration of these rules.
7.2.3. 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 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:
Always T_TriggerData.
Describes the event for which the function is called. You can use the following macros to examine tg_event:
Returns true if the trigger fired before the operation.
Returns true if the trigger fired after the operation.
Returns true if the trigger fired instead of the operation.
Returns true if the trigger fired for a row-level event.
Returns true if the trigger fired for a statement-level event.
Returns true if the trigger was fired by an INSERT command.
Returns true if the trigger was fired by an UPDATE command.
Returns true if the trigger was fired by a DELETE command.
Returns true if the trigger was fired by a TRUNCATE command.
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).
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.
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.
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.
The slot containing tg_trigtuple, or a NULL pointer if there is no such tuple.
The slot containing tg_newtuple, or a NULL pointer if there is no such tuple.
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.
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.
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-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.
7.2.4. A Complete Trigger Example
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 "fmgr.h"
#include "executor/spi.h" /* this is what you need to work with SPI */
#include "commands/trigger.h" /* ... triggers ... */
#include "utils/rel.h" /* ... and relations */
PG_MODULE_MAGIC;
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(ERROR, "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(ERROR, "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 (see dfunc), 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 FUNCTION trigf();
CREATE TRIGGER tafter AFTER INSERT OR UPDATE OR DELETE ON ttest
FOR EACH ROW EXECUTE FUNCTION 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.