CREATE TRIGGER

transition tables

ephemeral named relation

CREATE TRIGGER

7

SQL - Language Statements

CREATE TRIGGER

define a new trigger

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`     MDASH                        Tables                      
  `AFTER`        `INSERT`/`UPDATE`/`DELETE`   Tables and foreign tables   Tables, views, and foreign tables
  `TRUNCATE`     MDASH                        Tables                      
  `INSTEAD OF`   `INSERT`/`UPDATE`/`DELETE`   Views                       MDASH
  `TRUNCATE`     MDASH                        MDASH                       

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.trigger constraint trigger This is the same as a
regular trigger except that the timing of the trigger firing can be
adjusted using [`SET CONSTRAINTS`](#sql-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 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 [???](#triggers) 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 MDASH 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 [???](#sql-createtable)
    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 {#sql-createtrigger-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`](#sql-droptrigger) 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
[???](#functions-trigger).

Examples {#sql-createtrigger-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();

[???](#trigger-example) contains a complete example of a trigger
function written in C.

Compatibility {#sql-createtrigger-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
========
