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NAME

       CREATE_TRIGGER - define a new trigger

SYNOPSIS

       CREATE [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
           ON table
           [ FROM referenced_table_name ]
           { NOT DEFERRABLE | [ DEFERRABLE ] { INITIALLY IMMEDIATE | INITIALLY DEFERRED } }
           [ FOR [ EACH ] { ROW | STATEMENT } ]
           [ WHEN ( condition ) ]
           EXECUTE PROCEDURE function_name ( arguments )

       where event can be one of:

           INSERT
           UPDATE [ OF column_name [, ... ] ]
           DELETE
           TRUNCATE

DESCRIPTION

       CREATE TRIGGER creates a new trigger. The trigger will be associated
       with the specified table or view and will execute the specified
       function function_name when certain events occur.

       The trigger can be specified to fire 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); 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 (for INSERT and UPDATE 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 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).

       Triggers that are specified to fire INSTEAD OF the trigger event must
       be marked FOR EACH ROW, and can only be defined on views.  BEFORE and
       AFTER triggers on a view must be marked as FOR EACH STATEMENT.

       In addition, triggers may be defined to fire for TRUNCATE, though only
       FOR EACH STATEMENT.

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

       +-----------+----------------------+-----------+------------------+
       |When       | Event                | Row-level | Statement-level  |
       +-----------+----------------------+-----------+------------------+
       |           | INSERT/UPDATE/DELETE |  Tables   | Tables and views |
       |  BEFORE   +----------------------+-----------+------------------+
       |           |       TRUNCATE       |    --     |      Tables      |
       +-----------+----------------------+-----------+------------------+
       |           | INSERT/UPDATE/DELETE |  Tables   | Tables and views |
       |  AFTER    +----------------------+-----------+------------------+
       |           |       TRUNCATE       |    --     |      Tables      |
       +-----------+----------------------+-----------+------------------+
       |           | INSERT/UPDATE/DELETE |   Views   |        --        |
       |INSTEAD OF +----------------------+-----------+------------------+
       |           |       TRUNCATE       |    --     |        --        |
       +-----------+----------------------+-----------+------------------+

       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.
       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.

PARAMETERS

       name
           The 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 trigger's behavior using SET CONSTRAINTS.

       BEFORE, AFTER, INSTEAD OF
           Determines whether the function is called before, after, or instead
           of the event. A constraint trigger can only be specified as AFTER.

       event
           One 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
           The name (optionally schema-qualified) of the table or view the
           trigger is for.

       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. This can only be specified for
           constraint triggers.

       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. This can only be specified for constraint triggers.

       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. Constraint triggers can only be specified FOR EACH ROW.

       condition
           A 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_name
           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 to find out how these arguments can be accessed 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.

       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 command's SET 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 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 column's 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 trigger's WHEN condition is not allowed to
       examine the system columns of the NEW row (such as oid), because those
       won't 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 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.

       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

       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.

COMPATIBILITY

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

SEE ALSO

       CREATE FUNCTION (CREATE_FUNCTION(7)), ALTER TRIGGER (ALTER_TRIGGER(7)),
       DROP TRIGGER (DROP_TRIGGER(7)), SET CONSTRAINTS (SET_CONSTRAINTS(7))