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CREATE TRIGGER

CREATE TRIGGER — define a new trigger

Synopsis

CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGERname { BEFORE | AFTER | INSTEAD OF } {event [ OR ... ] }    ONtable_name    [ FROMreferenced_table_name ]    [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]    [ REFERENCING { { OLD | NEW } TABLE [ AS ]transition_relation_name } [ ... ] ]    [ FOR [ EACH ] { ROW | STATEMENT } ]    [ WHEN (condition ) ]    EXECUTE { FUNCTION | PROCEDURE }function_name (arguments )whereevent can be one of:    INSERT    UPDATE [ OFcolumn_name [, ... ] ]    DELETE    TRUNCATE

Description

CREATE TRIGGER creates a new trigger.CREATE OR REPLACE TRIGGER will either create a new trigger, or replace an existing trigger. The trigger will be associated with the specified table, view, or foreign table and will execute the specified functionfunction_name when certain operations are performed on that table.

To replace the current definition of an existing trigger, useCREATE OR REPLACE TRIGGER, specifying the existing trigger's name and parent table. All other properties are replaced.

The trigger can be specified to fire before the operation is attempted on a row (before constraints are checked and theINSERT,UPDATE, orDELETE is attempted); or after the operation has completed (after constraints are checked and theINSERT,UPDATE, orDELETE has completed); or instead of the operation (in the case of inserts, updates or deletes on a view). If the trigger fires before or instead of the event, the trigger can skip the operation for the current row, or change the row being inserted (forINSERT andUPDATE operations only). If the trigger fires after the event, all changes, including the effects of other triggers, are“visible” to the trigger.

A trigger that is markedFOR EACH ROW is called once for every row that the operation modifies. For example, aDELETE that affects 10 rows will cause anyON DELETE triggers on the target relation to be called 10 separate times, once for each deleted row. In contrast, a trigger that is markedFOR EACH STATEMENT only executes once for any given operation, regardless of how many rows it modifies (in particular, an operation that modifies zero rows will still result in the execution of any applicableFOR EACH STATEMENT triggers).

Triggers that are specified to fireINSTEAD OF the trigger event must be markedFOR EACH ROW, and can only be defined on views.BEFORE andAFTER triggers on a view must be marked asFOR EACH STATEMENT.

In addition, triggers may be defined to fire forTRUNCATE, though onlyFOR EACH STATEMENT.

The following table summarizes which types of triggers may be used on tables, views, and foreign tables:

Also, a trigger definition can specify a BooleanWHEN condition, which will be tested to see whether the trigger should be fired. In row-level triggers theWHEN condition can examine the old and/or new values of columns of the row. Statement-level triggers can also haveWHEN conditions, although the feature is not so useful for them since the condition cannot refer to any values in the table.

If multiple triggers of the same kind are defined for the same event, they will be fired in alphabetical order by name.

When theCONSTRAINT option is specified, this command creates aconstraint trigger. This is the same as a regular trigger except that the timing of the trigger firing can be adjusted usingSET CONSTRAINTS. Constraint triggers must beAFTER ROW triggers on plain tables (not foreign tables). They can be fired either at the end of the statement causing the triggering event, or at the end of the containing transaction; in the latter case they are said to bedeferred. A pending deferred-trigger firing can also be forced to happen immediately by usingSET CONSTRAINTS. Constraint triggers are expected to raise an exception when the constraints they implement are violated.

TheREFERENCING option enables collection oftransition relations, which are row sets that include all of the rows inserted, deleted, or modified by the current SQL statement. This feature lets the trigger see a global view of what the statement did, not just one row at a time. This option is only allowed for anAFTER trigger that is not a constraint trigger; also, if the trigger is anUPDATE trigger, it must not specify acolumn_name list.OLD TABLE may only be specified once, and only for a trigger that can fire onUPDATE orDELETE; it creates a transition relation containing thebefore-images of all rows updated or deleted by the statement. Similarly,NEW TABLE may only be specified once, and only for a trigger that can fire onUPDATE orINSERT; it creates a transition relation containing theafter-images of all rows updated or inserted by the statement.

SELECT does not modify any rows so you cannot createSELECT triggers. Rules and views may provide workable solutions to problems that seem to needSELECT triggers.

Refer toChapter 37 for more information about triggers.

Parameters

Notes#

To create or replace a trigger on a table, the user must have theTRIGGER privilege on the table. The user must also haveEXECUTE privilege on the trigger function.

UseDROP TRIGGER to remove a trigger.

Creating a row-level trigger on a partitioned table will cause an identical“clone” trigger to be created on each of its existing partitions; and any partitions created or attached later will have an identical trigger, too. If there is a conflictingly-named trigger on a child partition already, an error occurs unlessCREATE OR REPLACE TRIGGER is used, in which case that trigger is replaced with a clone trigger. When a partition is detached from its parent, its clone triggers are removed.

A column-specific trigger (one defined using theUPDATE OFcolumn_name syntax) will fire when any of its columns are listed as targets in theUPDATE command'sSET list. It is possible for a column's value to change even when the trigger is not fired, because changes made to the row's contents byBEFORE UPDATE triggers are not considered. Conversely, a command such asUPDATE ... SET x = x ... will fire a trigger on columnx, even though the column's value did not change.

In aBEFORE trigger, theWHEN condition is evaluated just before the function is or would be executed, so usingWHEN is not materially different from testing the same condition at the beginning of the trigger function. Note in particular that theNEW row seen by the condition is the current value, as possibly modified by earlier triggers. Also, aBEFORE trigger'sWHEN condition is not allowed to examine the system columns of theNEW row (such asctid), because those won't have been set yet.

In anAFTER trigger, theWHEN 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 anAFTER trigger'sWHEN 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.

In some cases it is possible for a single SQL command to fire more than one kind of trigger. For instance anINSERT with anON CONFLICT DO UPDATE clause may cause both insert and update operations, so it will fire both kinds of triggers as needed. The transition relations supplied to triggers are specific to their event type; thus anINSERT trigger will see only the inserted rows, while anUPDATE trigger will see only the updated rows.

Row updates or deletions caused by foreign-key enforcement actions, such asON UPDATE CASCADE orON DELETE SET NULL, are treated as part of the SQL command that caused them (note that such actions are never deferred). Relevant triggers on the affected table will be fired, so that this provides another way in which an SQL command might fire triggers not directly matching its type. In simple cases, triggers that request transition relations will see all changes caused in their table by a single original SQL command as a single transition relation. However, there are cases in which the presence of anAFTER ROW trigger that requests transition relations will cause the foreign-key enforcement actions triggered by a single SQL command to be split into multiple steps, each with its own transition relation(s). In such cases, any statement-level triggers that are present will be fired once per creation of a transition relation set, ensuring that the triggers see each affected row in a transition relation once and only once.

Statement-level triggers on a view are fired only if the action on the view is handled by a row-levelINSTEAD OF trigger. If the action is handled by anINSTEAD rule, then whatever statements are emitted by the rule are executed in place of the original statement naming the view, so that the triggers that will be fired are those on tables named in the replacement statements. Similarly, if the view is automatically updatable, then the action is handled by automatically rewriting the statement into an action on the view's base table, so that the base table's statement-level triggers are the ones that are fired.

Modifying a partitioned table or a table with inheritance children fires statement-level triggers attached to the explicitly named table, but not statement-level triggers for its partitions or child tables. In contrast, row-level triggers are fired on the rows in affected partitions or child tables, even if they are not explicitly named in the query. If a statement-level trigger has been defined with transition relations named by aREFERENCING clause, then before and after images of rows are visible from all affected partitions or child tables. In the case of inheritance children, the row images include only columns that are present in the table that the trigger is attached to.

Currently, row-level triggers with transition relations cannot be defined on partitions or inheritance child tables. Also, triggers on partitioned tables may not beINSTEAD OF.

Currently, theOR REPLACE option is not supported for constraint triggers.

Replacing an existing trigger within a transaction that has already performed updating actions on the trigger's table is not recommended. Trigger firing decisions, or portions of firing decisions, that have already been made will not be reconsidered, so the effects could be surprising.

There are a few built-in trigger functions that can be used to solve common problems without having to write your own trigger code; seeSection 9.29.

Examples#

Execute the functioncheck_account_update whenever a row of the tableaccounts is about to be updated:

CREATE TRIGGER check_update    BEFORE UPDATE ON accounts    FOR EACH ROW    EXECUTE FUNCTION check_account_update();

Modify that trigger definition to only execute the function if columnbalance is specified as a target in theUPDATE command:

CREATE OR REPLACE TRIGGER check_update    BEFORE UPDATE OF balance ON accounts    FOR EACH ROW    EXECUTE FUNCTION check_account_update();

This form only executes the function if columnbalance has in fact changed value:

CREATE TRIGGER check_update    BEFORE UPDATE ON accounts    FOR EACH ROW    WHEN (OLD.balance IS DISTINCT FROM NEW.balance)    EXECUTE FUNCTION check_account_update();

Call a function to log updates ofaccounts, but only if something changed:

CREATE TRIGGER log_update    AFTER UPDATE ON accounts    FOR EACH ROW    WHEN (OLD.* IS DISTINCT FROM NEW.*)    EXECUTE FUNCTION log_account_update();

Execute the functionview_insert_row for each row to insert rows into the tables underlying a view:

CREATE TRIGGER view_insert    INSTEAD OF INSERT ON my_view    FOR EACH ROW    EXECUTE FUNCTION view_insert_row();

Execute the functioncheck_transfer_balances_to_zero for each statement to confirm that thetransfer rows offset to a net of zero:

CREATE TRIGGER transfer_insert    AFTER INSERT ON transfer    REFERENCING NEW TABLE AS inserted    FOR EACH STATEMENT    EXECUTE FUNCTION check_transfer_balances_to_zero();

Execute the functioncheck_matching_pairs for each row to confirm that changes are made to matching pairs at the same time (by the same statement):

CREATE TRIGGER paired_items_update    AFTER UPDATE ON paired_items    REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab    FOR EACH ROW    EXECUTE FUNCTION check_matching_pairs();

Section 37.4 contains a complete example of a trigger function written in C.

Compatibility#

TheCREATE TRIGGER statement inPostgres Pro implements a subset of theSQL standard. The following functionalities are currently missing:

SQL specifies that multiple triggers should be fired in time-of-creation order.Postgres Pro uses name order, which was judged to be more convenient.

SQL specifies thatBEFORE DELETE triggers on cascaded deletes fireafter the cascadedDELETE completes. ThePostgres Pro behavior is forBEFORE DELETE to always fire before the delete action, even a cascading one. This is considered more consistent. There is also nonstandard behavior ifBEFORE triggers modify rows or prevent updates during an update that is caused by a referential action. This can lead to constraint violations or stored data that does not honor the referential constraint.

The ability to specify multiple actions for a single trigger usingOR is aPostgres Pro extension of the SQL standard.

The ability to fire triggers forTRUNCATE is aPostgres Pro extension of the SQL standard, as is the ability to define statement-level triggers on views.

CREATE CONSTRAINT TRIGGER is aPostgres Pro extension of theSQL standard. So is theOR REPLACE option.

See Also