CREATE TRIGGER

Name

CREATE TRIGGER -- define a new trigger

Synopsis

CREATE TRIGGER name { BEFORE | AFTER } { event [OR ...] }
     ON table FOR EACH { ROW | STATEMENT }
     EXECUTE PROCEDURE func ( arguments )

Inputs

name: The name to give the new trigger. This must be distinct from the name of any other trigger for the same table.
event: One of INSERT, DELETE or UPDATE.
table: The name (optionally schema-qualified) of the table the trigger is for.
func: A user-supplied function that is declared as taking no arguments and returning type trigger.
arguments: An optional comma-separated list of arguments to be provided to the function when the trigger is executed, along with the standard trigger data such as old and new tuple contents. The arguments are literal string constants. Simple names and numeric constants may be written here too, but they will all be converted to strings.

Outputs

CREATE TRIGGER: This message is returned if the trigger is successfully created.

Description

CREATE TRIGGER will enter a new trigger into the current data base. The trigger will be associated with the relation table and will execute the specified function func.

The trigger can be specified to fire either before BEFORE the operation is attempted on a tuple (before constraints are checked and the INSERT, UPDATE or DELETE is attempted) or AFTER the operation has been attempted (e.g., after constraints are checked and the INSERT, UPDATE or DELETE has completed). If the trigger fires before the event, the trigger may skip the operation for the current tuple, or change the tuple being inserted (for INSERT and UPDATE operations only). If the trigger fires after the event, all changes, including the last insertion, update, or deletion, are "visible" to the trigger.

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

SELECT does not modify any rows so you can not create SELECT triggers. Rules and views are more appropriate in such cases.

Refer to the chapters on SPI and Triggers in the PostgreSQL Programmer's Guide for more information.

Notes

To create a trigger on a table, the user must have the TRIGGER privilege on the table.

In PostgreSQL versions before 7.3, it was necessary to declare trigger functions as returning the placeholder type opaque, rather than trigger. To support loading of old dump files, CREATE TRIGGER will accept a function declared as returning opaque, but it will issue a NOTICE and change the function's declared return type to trigger.

As of the current release, STATEMENT triggers are not implemented.

Refer to the DROP TRIGGER command for information on how to remove triggers.

Examples

Check if the specified distributor code exists in the distributors table before appending or updating a row in the table films:

CREATE TRIGGER if_dist_exists
     BEFORE INSERT OR UPDATE ON films FOR EACH ROW
     EXECUTE PROCEDURE check_primary_key ('did', 'distributors', 'did');

Before cancelling a distributor or updating its code, remove every reference to the table films:

CREATE TRIGGER if_film_exists 
     BEFORE DELETE OR UPDATE ON distributors FOR EACH ROW
     EXECUTE PROCEDURE check_foreign_key (1, 'CASCADE', 'did', 'films', 'did');

The second example can also be done by using a foreign key, constraint as in:

CREATE TABLE distributors (
     did      DECIMAL(3),
     name     VARCHAR(40),
     CONSTRAINT if_film_exists
     FOREIGN KEY(did) REFERENCES films
     ON UPDATE CASCADE ON DELETE CASCADE  
 );

Compatibility

SQL92

There is no CREATE TRIGGER statement in SQL92.

SQL99

The CREATE TRIGGER statement in PostgreSQL implements a subset of the SQL99 standard. The following functionality is missing:

SQL99 allows triggers to fire on updates to specific columns (e.g., AFTER UPDATE OF col1, col2).
SQL99 allows you to define aliases for the "old" and "new" rows or tables for use in the definition of the triggered action (e.g., CREATE TRIGGER ... ON tablename REFERENCING OLD ROW AS somename NEW ROW AS othername ...). Since PostgreSQL allows trigger procedures to be written in any number of user-defined languages, access to the data is handled in a language-specific way.
PostgreSQL only has row-level triggers, no statement-level triggers.
PostgreSQL only allows the execution of a stored procedure for the triggered action. SQL99 allows the execution of a number of other SQL commands, such as CREATE TABLE as triggered action. This limitation is not hard to work around by creating a stored procedure that executes these commands.

SQL99 specifies that multiple triggers should be fired in time-of-creation order. PostgreSQL uses name order, which was judged more convenient to work with.