Convert HTML docs to more streamlined TXT

[ai-pg] / full-docs / txt / sql-createtrigger.txt

CREATE TRIGGER

   CREATE TRIGGER — define a new trigger

Synopsis

CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF
} { event [ OR ... ] }
    ON table_name
    [ FROM referenced_table_name ]
    [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED
 ] ]
    [ 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:

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

Description

   CREATE TRIGGER creates a new trigger. CREATE OR REPLACE TRIGGER will
   either create a new trigger, or replace an existing 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.

   To replace the current definition of an existing trigger, use CREATE OR
   REPLACE TRIGGER, specifying the existing trigger's name and parent
   table. All other properties are replaced.

   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, views, and foreign tables:
   When Event Row-level Statement-level
   BEFORE INSERT/UPDATE/DELETE Tables and foreign tables Tables, views,
   and foreign tables
   TRUNCATE — Tables and foreign tables
   AFTER INSERT/UPDATE/DELETE Tables and foreign tables Tables, views, and
   foreign tables
   TRUNCATE — Tables and foreign tables
   INSTEAD OF INSERT/UPDATE/DELETE Views —
   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.
   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 on a plain table
   (not a foreign table). The trigger should not be a constraint trigger.
   Also, if the trigger is an UPDATE trigger, it must not specify a
   column_name list when using this option. 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 37 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, 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 or if one of the
          listed columns is a generated column that depends on a column
          that is the target of the UPDATE.

          INSTEAD OF UPDATE events do not allow a list of columns. A
          column list cannot be specified when requesting transition
          relations, either.

   table_name
          The name (optionally schema-qualified) of the table, view, or
          foreign table 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
          documentation for details of these constraint options. This can
          only be specified for constraint triggers.

   REFERENCING
          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.

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

   FOR EACH ROW
          FOR EACH STATEMENT
          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.

   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.

          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.

   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 or replace 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 to remove a trigger.

   Creating a row-level trigger on a partitioned table will cause an
   identical “clone” trigger to be created on each of its existing
   partitions; and any partitions created or attached later will have an
   identical trigger, too. If there is a conflictingly-named trigger on a
   child partition already, an error occurs unless CREATE OR REPLACE
   TRIGGER is used, in which case that trigger is replaced with a clone
   trigger. When a partition is detached from its parent, its clone
   triggers are removed.

   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 ctid), 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 an 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.

   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. Also, triggers on
   partitioned tables may not be INSTEAD OF.

   Currently, the OR REPLACE option is not supported for constraint
   triggers.

   Replacing an existing trigger within a transaction that has already
   performed updating actions on the trigger's table is not recommended.
   Trigger firing decisions, or portions of firing decisions, that have
   already been made will not be reconsidered, so the effects could be
   surprising.

   There are a few built-in trigger functions that can be used to solve
   common problems without having to write your own trigger code; see
   Section 9.29.

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 FUNCTION check_account_update();

   Modify that trigger definition to only execute the function if column
   balance is specified as a target in the UPDATE command:
CREATE OR REPLACE 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 37.4 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 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. So is the OR REPLACE option.

See Also

   ALTER TRIGGER, DROP TRIGGER, CREATE FUNCTION, SET CONSTRAINTS