37.1. Overview of Trigger Behavior
A trigger is a specification that the database should automatically execute a particular function whenever a certain type of operation is performed. Triggers can be attached to tables, views, and foreign tables.
On tables and foreign tables, triggers can be defined to execute either before or after any
DELETE operation, either once per modified row, or once per SQL statement. If an
INSERT contains an
ON CONFLICT DO UPDATE clause, it is possible that the effects of a BEFORE insert trigger and a BEFORE update trigger can both be applied together, if a reference to an
EXCLUDED column appears.
UPDATE triggers can moreover be set to fire only if certain columns are mentioned in the
SET clause of the
UPDATE statement. Triggers can also fire for
TRUNCATE statements. If a trigger event occurs, the trigger's function is called at the appropriate time to handle the event. Foreign tables do not support the TRUNCATE statement at all.
On views, triggers can be defined to execute instead of
DELETE operations. Such
INSTEAD OF triggers are fired once for each row that needs to be modified in the view. It is the responsibility of the trigger's function to perform the necessary modifications to the view's underlying base table(s) and, where appropriate, return the modified row as it will appear in the view. Triggers on views can also be defined to execute once per SQL statement, before or after
DELETE operations. However, such triggers are fired only if there is also an
INSTEAD OF trigger on the view. Otherwise, any statement targeting the view must be rewritten into a statement affecting its underlying base table(s), and then the triggers that will be fired are the ones attached to the base table(s).
The trigger function must be defined before the trigger itself can be created. The trigger function must be declared as a function taking no arguments and returning type
trigger. (The trigger function receives its input through a specially-passed
TriggerData structure, not in the form of ordinary function arguments.)
Once a suitable trigger function has been created, the trigger is established with CREATE TRIGGER. The same trigger function can be used for multiple triggers.
Postgres Pro offers both per-row triggers and per-statement triggers. With a per-row trigger, the trigger function is invoked once for each row that is affected by the statement that fired the trigger. In contrast, a per-statement trigger is invoked only once when an appropriate statement is executed, regardless of the number of rows affected by that statement. In particular, a statement that affects zero rows will still result in the execution of any applicable per-statement triggers. These two types of triggers are sometimes called row-level triggers and statement-level triggers, respectively. Triggers on
TRUNCATE may only be defined at statement level. On views, triggers that fire before or after may only be defined at statement level, while triggers that fire instead of an
DELETE may only be defined at row level.
Triggers are also classified according to whether they fire before, after, or instead of the operation. These are referred to as
AFTER triggers, and
INSTEAD OF triggers respectively. Statement-level
BEFORE triggers naturally fire before the statement starts to do anything, while statement-level
AFTER triggers fire at the very end of the statement. These types of triggers may be defined on tables or views. Row-level
BEFORE triggers fire immediately before a particular row is operated on, while row-level
AFTER triggers fire at the end of the statement (but before any statement-level
AFTER triggers). These types of triggers may only be defined on tables and foreign tables. Row-level
INSTEAD OF triggers may only be defined on views, and fire immediately as each row in the view is identified as needing to be operated on.
INSERT contains an
ON CONFLICT DO UPDATE clause, it is possible that the effects of all row-level
INSERT triggers and all row-level
UPDATE triggers can both be applied in a way that is apparent from the final state of the updated row, if an
EXCLUDED column is referenced. There need not be an
EXCLUDED column reference for both sets of row-level
BEFORE triggers to execute, though. The possibility of surprising outcomes should be considered when there are both
UPDATE row-level triggers that both affect a row being inserted/updated (this can still be problematic if the modifications are more or less equivalent if they're not also idempotent). Note that statement-level
UPDATE triggers are executed when
ON CONFLICT DO UPDATE is specified, regardless of whether or not any rows were affected by the
UPDATE (and regardless of whether the alternative
UPDATE path was ever taken). An
INSERT with an
ON CONFLICT DO UPDATE clause will execute statement-level
INSERT triggers first, then statement-level
UPDATE triggers, followed by statement-level
UPDATE triggers and finally statement-level
Trigger functions invoked by per-statement triggers should always return
NULL. Trigger functions invoked by per-row triggers can return a table row (a value of type
HeapTuple) to the calling executor, if they choose. A row-level trigger fired before an operation has the following choices:
It can return
NULLto skip the operation for the current row. This instructs the executor to not perform the row-level operation that invoked the trigger (the insertion, modification, or deletion of a particular table row).
UPDATEtriggers only, the returned row becomes the row that will be inserted or will replace the row being updated. This allows the trigger function to modify the row being inserted or updated.
BEFORE trigger that does not intend to cause either of these behaviors must be careful to return as its result the same row that was passed in (that is, the
NEW row for
UPDATE triggers, the
OLD row for
INSTEAD OF trigger should either return
NULL to indicate that it did not modify any data from the view's underlying base tables, or it should return the view row that was passed in (the
NEW row for
UPDATE operations, or the
OLD row for
DELETE operations). A nonnull return value is used to signal that the trigger performed the necessary data modifications in the view. This will cause the count of the number of rows affected by the command to be incremented. For
UPDATE operations only, the trigger may modify the
NEW row before returning it. This will change the data returned by
INSERT RETURNING or
UPDATE RETURNING, and is useful when the view will not show exactly the same data that was provided.
The return value is ignored for row-level triggers fired after an operation, and so they can return
If more than one trigger is defined for the same event on the same relation, the triggers will be fired in alphabetical order by trigger name. In the case of
INSTEAD OF triggers, the possibly-modified row returned by each trigger becomes the input to the next trigger. If any
INSTEAD OF trigger returns
NULL, the operation is abandoned for that row and subsequent triggers are not fired (for that row).
A trigger definition can also 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.) 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. However, 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
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.
INSTEAD OF triggers do not support
BEFORE triggers are used for checking or modifying the data that will be inserted or updated. For example, a
BEFORE trigger might be used to insert the current time into a
timestamp column, or to check that two elements of the row are consistent. Row-level
AFTER triggers are most sensibly used to propagate the updates to other tables, or make consistency checks against other tables. The reason for this division of labor is that an
AFTER trigger can be certain it is seeing the final value of the row, while a
BEFORE trigger cannot; there might be other
BEFORE triggers firing after it. If you have no specific reason to make a trigger
BEFORE case is more efficient, since the information about the operation doesn't have to be saved until end of statement.
If a trigger function executes SQL commands then these commands might fire triggers again. This is known as cascading triggers. There is no direct limitation on the number of cascade levels. It is possible for cascades to cause a recursive invocation of the same trigger; for example, an
INSERT trigger might execute a command that inserts an additional row into the same table, causing the
INSERT trigger to be fired again. It is the trigger programmer's responsibility to avoid infinite recursion in such scenarios.
When a trigger is being defined, arguments can be specified for it. The purpose of including arguments in the trigger definition is to allow different triggers with similar requirements to call the same function. As an example, there could be a generalized trigger function that takes as its arguments two column names and puts the current user in one and the current time stamp in the other. Properly written, this trigger function would be independent of the specific table it is triggering on. So the same function could be used for
INSERT events on any table with suitable columns, to automatically track creation of records in a transaction table for example. It could also be used to track last-update events if defined as an
Each programming language that supports triggers has its own method for making the trigger input data available to the trigger function. This input data includes the type of trigger event (e.g.,
UPDATE) as well as any arguments that were listed in
CREATE TRIGGER. For a row-level trigger, the input data also includes the
NEW row for
UPDATE triggers, and/or the
OLD row for
DELETE triggers. Statement-level triggers do not currently have any way to examine the individual row(s) modified by the statement.