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       CREATE_TRIGGER - define a new trigger


       CREATE [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
           ON table
           [ FROM referenced_table_name ]
           [ FOR [ EACH ] { ROW | STATEMENT } ]
           [ WHEN ( condition ) ]
           EXECUTE PROCEDURE function_name ( arguments )

       where event can be one of:

           UPDATE [ OF column_name [, ... ] ]


       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

       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 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

       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:

       │WhenEventRow-levelStatement-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.


           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.

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

           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.

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

           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.

           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.

           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.

           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

           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.

           A user-supplied function that is declared as taking no arguments and returning type
           trigger, which is executed when the trigger fires.

           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.


       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 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.


       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 functionality is currently missing:

       ·   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.

       ·   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.