|BEFORE||INSERT/UPDATE/DELETE||Tables and foreign tables||Tables, views, and foreign tables|
|AFTER||INSERT/UPDATE/DELETE||Tables and foreign tables||Tables, views, and foreign tables|
Also, a trigger definition can 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 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 the CONSTRAINT option is specified, this command creates a constraint trigger. This is the same as a regular trigger except that the timing of the trigger firing can be adjusted using SET CONSTRAINTS (SET_CONSTRAINTS(7)). Constraint triggers must be AFTER ROW triggers on 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 be deferred. A pending deferred-trigger firing can also be forced to happen immediately by using SET CONSTRAINTS. Constraint triggers are expected to raise an exception when the constraints they implement are violated.
SELECT does not modify any rows so you cannot create SELECT triggers. Rules and views are more appropriate in such cases.
Refer to Chapter 36, Triggers, in the documentation for more information about triggers.
nameThe name to give the new trigger. This must be distinct from the name of any other trigger for the same table. The name cannot be schema-qualified the trigger inherits the schema of its table. For a constraint trigger, this is also the name to use when modifying the triggers behavior using SET CONSTRAINTS.
INSTEAD OFDetermines whether the function is called before, after, or instead of the event. A constraint trigger can only be specified as AFTER.
eventOne of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the event that will fire the trigger. Multiple events can be specified using OR.
For UPDATE events, it is possible to specify a list of columns using this syntax:
UPDATE OF column_name1 [, column_name2 ... ]
The trigger will only fire if at least one of the listed columns is mentioned as a target of the UPDATE command.
INSTEAD OF UPDATE events do not support lists of columns.
table_nameThe name (optionally schema-qualified) of the table, view, or foreign table the trigger is for.
referenced_table_nameThe (possibly schema-qualified) name of another table referenced by the constraint. This option is used for foreign-key constraints and is not recommended for general use. This can only be specified for constraint triggers.
INITIALLY DEFERREDThe default timing of the trigger. See the CREATE TABLE (CREATE_TABLE(7)) documentation for details of these constraint options. This can only be specified for constraint triggers.
FOR EACH ROW
FOR EACH STATEMENTThis specifies whether the trigger procedure should be fired once for every row affected by the trigger event, or just once per SQL statement. If neither is specified, FOR EACH STATEMENT is the default. Constraint triggers can only be specified FOR EACH ROW.
conditionA Boolean expression that determines whether the trigger function will actually be executed. If WHEN is specified, the function will only be called if the condition returns true. In FOR EACH ROW triggers, the WHEN condition can refer to columns of the old and/or new row values by writing OLD.column_name or NEW.column_name respectively. Of course, INSERT triggers cannot refer to OLD and DELETE triggers cannot refer to NEW.
INSTEAD OF triggers do not support WHEN conditions.
Currently, WHEN expressions cannot contain subqueries.
Note that for constraint triggers, evaluation of the WHEN condition is not deferred, but occurs immediately after the row update operation is performed. If the condition does not evaluate to true then the trigger is not queued for deferred execution.
function_nameA user-supplied function that is declared as taking no arguments and returning type trigger, which is executed when the trigger fires.
argumentsAn optional comma-separated list of arguments to be provided to the function when the trigger is executed. The arguments are literal string constants. Simple names and numeric constants can be written here, too, but they will all be converted to strings. Please check the description of the implementation language of the trigger function to find out how these arguments can be accessed within the function; it might be different from normal function arguments.
To create a trigger on a table, the user must have the TRIGGER privilege on the table. The user must also have EXECUTE privilege on the trigger function.
Use DROP TRIGGER (DROP_TRIGGER(7)) to remove a trigger.
A column-specific trigger (one defined using the UPDATE OF column_name syntax) will fire when any of its columns are listed as targets in the UPDATE commands SET list. It is possible for a columns value to change even when the trigger is not fired, because changes made to the rows contents by BEFORE UPDATE triggers are not considered. Conversely, a command such as UPDATE ... SET x = x ... will fire a trigger on column x, even though the columns value did not change.
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. Note in particular that the NEW row seen by the condition is the current value, as possibly modified by earlier triggers. Also, a BEFORE triggers WHEN condition is not allowed to examine the system columns of the NEW row (such as oid), because those wont have been set yet.
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 triggers 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.
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 functions declared return type to trigger.
Execute the function check_account_update whenever a row of the table accounts is about to be updated:
CREATE TRIGGER check_update BEFORE UPDATE ON accounts FOR EACH ROW EXECUTE PROCEDURE check_account_update();
The same, but only execute the function if column balance is specified as a target in the UPDATE command:
CREATE TRIGGER check_update BEFORE UPDATE OF balance ON accounts FOR EACH ROW EXECUTE PROCEDURE check_account_update();
This form only executes the function if column balance 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 PROCEDURE check_account_update();
Call a function to log updates of accounts, but only if something changed:
CREATE TRIGGER log_update AFTER UPDATE ON accounts FOR EACH ROW WHEN (OLD.* IS DISTINCT FROM NEW.*) EXECUTE PROCEDURE log_account_update();
Execute the function view_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 PROCEDURE view_insert_row();
Section 36.4, A Complete Trigger Example, in the documentation contains a complete example of a trigger function written in C.
The CREATE TRIGGER statement in PostgreSQL implements a subset of the SQL standard. The following functionalities are currently missing:
o SQL 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.
o PostgreSQL does not allow the old and new tables to be referenced in statement-level triggers, i.e., the tables that contain all the old and/or new rows, which are referred to by the OLD TABLE and NEW TABLE clauses in the SQL standard.
o PostgreSQL only allows the execution of a user-defined function for the triggered action. The standard allows the execution of a number of other SQL commands, such as CREATE TABLE, as the triggered action. This limitation is not hard to work around by creating a user-defined function that executes the desired commands.
SQL specifies that multiple triggers should be fired in time-of-creation order. PostgreSQL uses name order, which was judged to be more convenient.
SQL specifies that BEFORE DELETE triggers on cascaded deletes fire after the cascaded DELETE completes. The PostgreSQL behavior is for BEFORE DELETE to always fire before the delete action, even a cascading one. This is considered more consistent. There is also nonstandard behavior if BEFORE 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 using OR is a PostgreSQL extension of the SQL standard.
The ability to fire triggers for TRUNCATE is a PostgreSQL extension of the SQL standard, as is the ability to define statement-level triggers on views.
CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL standard.
ALTER TRIGGER (ALTER_TRIGGER(7)), DROP TRIGGER (DROP_TRIGGER(7)), CREATE FUNCTION (CREATE_FUNCTION(7)), SET CONSTRAINTS (SET_CONSTRAINTS(7))
|PostgreSQL 9.5.2||CREATE TRIGGER (7)||2016|