Ubuntu Manpage: CREATE_TRIGGER - define a new trigger

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NAME

       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    │
       ├────────────┼──────────────────────┼────────────────────┼────────────────────┤
       │            │ 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.
       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 39 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 (CREATE_TABLE(7)) 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 9.28.

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 39.4 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.