CREATE CONSTRAINT TRIGGER(7) SQL Commands CREATE CONSTRAINT TRIGGER(7)NAME
CREATE CONSTRAINT TRIGGER - define a new constraint trigger
SYNOPSIS
CREATE CONSTRAINT TRIGGER name
AFTER event [ OR ... ]
ON table_name
[ FROM referenced_table_name ]
{ NOT DEFERRABLE | [ DEFERRABLE ] { INITIALLY IMMEDIATE | INITIALLY DEFERRED } }
FOR EACH ROW
EXECUTE PROCEDURE funcname ( arguments )
DESCRIPTION
CREATE CONSTRAINT TRIGGER 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. 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.
PARAMETERS
name The name of the constraint trigger. This is also the name to use when modifying the trigger's behavior using SET CONSTRAINTS. The
name cannot be schema-qualified -- the trigger inherits the schema of its table.
event One of INSERT, UPDATE, or DELETE; this specifies the event that will fire the trigger. Multiple events can be specified using OR.
table_name
The (possibly schema-qualified) name of the table in which the triggering events occur.
referenced_table_name
The (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.
DEFERRABLE
NOT DEFERRABLE
INITIALLY IMMEDIATE
INITIALLY DEFERRED
The default timing of the trigger. See the CREATE TABLE [create_table(7)] documentation for details of these constraint options.
funcname
The function to call when the trigger is fired. See CREATE TRIGGER [create_trigger(7)] for details.
arguments
Optional argument strings to pass to the trigger function. See CREATE TRIGGER [create_trigger(7)] for details.
COMPATIBILITY
CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL standard.
SEE ALSO
CREATE TRIGGER [create_trigger(7)], DROP TRIGGER [drop_trigger(7)], SET CONSTRAINTS [set_constraints(7)]
SQL - Language Statements 2010-05-14 CREATE CONSTRAINT TRIGGER(7)
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CREATE TRIGGER(7) SQL Commands CREATE TRIGGER(7)NAME
CREATE TRIGGER - define a new trigger
SYNOPSIS
CREATE TRIGGER name { BEFORE | AFTER } { event [ OR ... ] }
ON table [ FOR [ EACH ] { ROW | STATEMENT } ]
EXECUTE PROCEDURE funcname ( arguments )
DESCRIPTION
CREATE TRIGGER creates a new trigger. The trigger will be associated with the specified table and will execute the specified function func-
name when certain events occur.
The trigger can be specified to fire either before the operation is attempted on a row (before constraints are checked and the INSERT,
UPDATE, or DELETE is attempted) or after the operation has completed (after constraints are checked and the INSERT, UPDATE, or DELETE has
completed). If the trigger fires before the event, the trigger can skip the operation for the current row, or change the row being inserted
(for INSERT and UPDATE operations only). If the trigger fires after the event, all changes, including the last insertion, update, or dele-
tion, are ``visible'' to the trigger.
A trigger that is marked FOR EACH ROW is called once for every row that the operation modifies. For example, a DELETE that affects 10 rows
will cause any ON DELETE triggers on the target relation to be called 10 separate times, once for each deleted row. In contrast, a trigger
that is marked FOR 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 applicable FOR EACH STATEMENT triggers).
In addition, triggers may be defined to fire for a TRUNCATE, though only FOR EACH STATEMENT.
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 cannot create SELECT triggers. Rules and views are more appropriate in such cases.
Refer to in the documentation for more information about triggers.
PARAMETERS
name The name to give the new trigger. This must be distinct from the name of any other trigger for the same table.
BEFORE
AFTER Determines whether the function is called before or after the event.
event One of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the event that will fire the trigger. Multiple events can be specified
using OR.
table The name (optionally schema-qualified) of the table the trigger is for.
FOR EACH ROW
FOR EACH STATEMENT
This 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.
funcname
A user-supplied function that is declared as taking no arguments and returning type trigger, which is executed when the trigger
fires.
arguments
An 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 about how the trigger arguments are accessible within
the function; it might be different from normal function arguments.
NOTES
To create a trigger on a table, the user must have the TRIGGER privilege on the table.
Use DROP TRIGGER [drop_trigger(7)] to remove a trigger.
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.
EXAMPLES
in the documentation contains a complete example.
COMPATIBILITY
The CREATE TRIGGER statement in PostgreSQL implements a subset of the SQL standard. The following functionality is currently missing:
o SQL allows triggers to fire on updates to specific columns (e.g., AFTER UPDATE OF col1, col2).
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., CRE-
ATE 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 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 unpre-
dictable behavior when BEFORE triggers modify rows that are later to be modified by referential actions. This can lead to constraint viola-
tions 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.
SEE ALSO
CREATE FUNCTION [create_function(7)], ALTER TRIGGER [alter_trigger(7)], DROP TRIGGER [drop_trigger(7)]
SQL - Language Statements 2010-05-14 CREATE TRIGGER(7)