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       CREATE_TRIGGER - define a new trigger


       CREATE [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
           ON table_name
           [ FROM referenced_table_name ]
           [ REFERENCING { { OLD | NEW } TABLE [ AS ] transition_relation_name } [ ... ] ]
           [ FOR [ EACH ] { ROW | STATEMENT } ]
           [ WHEN ( condition ) ]
           EXECUTE { FUNCTION | 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, view, or foreign table and will execute the specified function function_name when
       certain operations are performed on that table.

       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, views, and
       foreign tables:

       │WhenEventRow-levelStatement-level    │
       │           │ INSERT/UPDATE/DELETE │ Tables and foreign │ Tables, views, and │
       │  BEFORE   │                      │ tables             │ foreign tables     │
       │           ├──────────────────────┼────────────────────┼────────────────────┤
       │           │       TRUNCATE       │         —          │       Tables       │
       │           │ INSERT/UPDATE/DELETE │ Tables and foreign │ Tables, views, and │
       │  AFTER    │                      │ tables             │ foreign tables     │
       │           ├──────────────────────┼────────────────────┼────────────────────┤
       │           │       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 on plain tables (not foreign 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.

       The REFERENCING option enables collection of transition relations, which are row sets that
       include all of the rows inserted, deleted, or modified by the current SQL statement. This
       feature lets the trigger see a global view of what the statement did, not just one row at
       a time. This option is only allowed for an AFTER trigger that is not a constraint trigger;
       also, if the trigger is an UPDATE trigger, it must not specify a column_name list.  OLD
       TABLE may only be specified once, and only for a trigger that can fire on UPDATE or
       DELETE; it creates a transition relation containing the before-images of all rows updated
       or deleted by the statement. Similarly, NEW TABLE may only be specified once, and only for
       a trigger that can fire on UPDATE or INSERT; it creates a transition relation containing
       the after-images of all rows updated or inserted by the statement.

       SELECT does not modify any rows so you cannot create SELECT triggers. Rules and views may
       provide workable solutions to problems that seem to need SELECT triggers.

       Refer to Chapter 39 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, except when transition
           relations are requested.

           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 allow a list of columns. A column list cannot be
           specified when requesting transition relations, either.

           The name (optionally schema-qualified) of the table, view, or foreign table 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 keyword immediately precedes the declaration of one or two relation names that
           provide access to the transition relations of the triggering statement.

       OLD TABLE
       NEW TABLE
           This clause indicates whether the following relation name is for the before-image
           transition relation or the after-image transition relation.

           The (unqualified) name to be used within the trigger for this transition relation.

           This specifies whether the trigger function 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.

           In the syntax of CREATE TRIGGER, the keywords FUNCTION and PROCEDURE are equivalent,
           but the referenced function must in any case be a function, not a procedure. The use
           of the keyword PROCEDURE here is historical and deprecated.

           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 some cases it is possible for a single SQL command to fire more than one kind of
       trigger. For instance an INSERT with an ON CONFLICT DO UPDATE clause may cause both insert
       and update operations, so it will fire both kinds of triggers as needed. The transition
       relations supplied to triggers are specific to their event type; thus an INSERT trigger
       will see only the inserted rows, while an UPDATE trigger will see only the updated rows.

       Row updates or deletions caused by foreign-key enforcement actions, such as ON UPDATE
       CASCADE or ON DELETE SET NULL, are treated as part of the SQL command that caused them
       (note that such actions are never deferred). Relevant triggers on the affected table will
       be fired, so that this provides another way in which a SQL command might fire triggers not
       directly matching its type. In simple cases, triggers that request transition relations
       will see all changes caused in their table by a single original SQL command as a single
       transition relation. However, there are cases in which the presence of an AFTER ROW
       trigger that requests transition relations will cause the foreign-key enforcement actions
       triggered by a single SQL command to be split into multiple steps, each with its own
       transition relation(s). In such cases, any statement-level triggers that are present will
       be fired once per creation of a transition relation set, ensuring that the triggers see
       each affected row in a transition relation once and only once.

       Statement-level triggers on a view are fired only if the action on the view is handled by
       a row-level INSTEAD OF trigger. If the action is handled by an INSTEAD rule, then whatever
       statements are emitted by the rule are executed in place of the original statement naming
       the view, so that the triggers that will be fired are those on tables named in the
       replacement statements. Similarly, if the view is automatically updatable, then the action
       is handled by automatically rewriting the statement into an action on the view's base
       table, so that the base table's statement-level triggers are the ones that are fired.

       Creating a row-level trigger on a partitioned table will cause identical triggers to be
       created in all its existing partitions; and any partitions created or attached later will
       contain an identical trigger, too. Triggers on partitioned tables may only be AFTER.

       Modifying a partitioned table or a table with inheritance children fires statement-level
       triggers attached to the explicitly named table, but not statement-level triggers for its
       partitions or child tables. In contrast, row-level triggers are fired on the rows in
       affected partitions or child tables, even if they are not explicitly named in the query.
       If a statement-level trigger has been defined with transition relations named by a
       REFERENCING clause, then before and after images of rows are visible from all affected
       partitions or child tables. In the case of inheritance children, the row images include
       only columns that are present in the table that the trigger is attached to. Currently,
       row-level triggers with transition relations cannot be defined on partitions or
       inheritance child tables.

       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 FUNCTION 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 FUNCTION 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 FUNCTION 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 FUNCTION 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 FUNCTION view_insert_row();

       Execute the function check_transfer_balances_to_zero for each statement to confirm that
       the transfer rows offset to a net of zero:

           CREATE TRIGGER transfer_insert
               AFTER INSERT ON transfer
               REFERENCING NEW TABLE AS inserted
               FOR EACH STATEMENT
               EXECUTE FUNCTION check_transfer_balances_to_zero();

       Execute the function check_matching_pairs for each row to confirm that changes are made to
       matching pairs at the same time (by the same statement):

           CREATE TRIGGER paired_items_update
               AFTER UPDATE ON paired_items
               REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
               FOR EACH ROW
               EXECUTE FUNCTION check_matching_pairs();

       Section 39.4 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:

       ·   While transition table names for AFTER triggers are specified using the REFERENCING
           clause in the standard way, the row variables used in FOR EACH ROW triggers may not be
           specified in a REFERENCING clause. They are available in a manner that is dependent on
           the language in which the trigger function is written, but is fixed for any one
           language. Some languages effectively behave as though there is a REFERENCING clause
           containing OLD ROW AS OLD NEW ROW AS NEW.

       ·   The standard allows transition tables to be used with column-specific UPDATE triggers,
           but then the set of rows that should be visible in the transition tables depends on
           the trigger's column list. This is not currently implemented by PostgreSQL.

       ·   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.