39.1. Overview of Trigger Behavior
Prev | Up | Chapter 39. Triggers | Home | Next

## 39.1. 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 (partitioned or not), views, and foreign tables.

On tables and foreign tables, triggers can be defined to execute either before or after any `INSERT`, `UPDATE`, or `DELETE` operation, either once per modified row, or once per SQL statement. `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.

On views, triggers can be defined to execute instead of `INSERT`, `UPDATE`, or `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 `INSERT`, `UPDATE`, or `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.

PostgreSQL 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, not per-row.

Triggers are also classified according to whether they fire before, after, or instead of the operation. These are referred to as `BEFORE` triggers, `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, views, or foreign tables. 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, not views. `INSTEAD OF` triggers may only be defined on views, and only at row level; they fire immediately as each row in the view is identified as needing to be operated on.

The execution of an `AFTER` trigger can be deferred to the end of the transaction, rather than the end of the statement, if it was defined as a constraint trigger. In all cases, a trigger is executed as part of the same transaction as the statement that triggered it, so if either the statement or the trigger causes an error, the effects of both will be rolled back.

A statement that targets a parent table in an inheritance or partitioning hierarchy does not cause the statement-level triggers of affected child tables to be fired; only the parent table's statement-level triggers are fired. However, row-level triggers of any affected child tables will be fired.

If an `INSERT` contains an `ON CONFLICT DO UPDATE` clause, it is possible that the effects of row-level `BEFORE` `INSERT` triggers and row-level `BEFORE` `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 `BEFORE` `INSERT` and `BEFORE` `UPDATE` row-level triggers that change a row being inserted/updated (this can be problematic even 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 `BEFORE` `INSERT` triggers first, then statement-level `BEFORE` `UPDATE` triggers, followed by statement-level `AFTER` `UPDATE` triggers and finally statement-level `AFTER` `INSERT` triggers.

If an `UPDATE` on a partitioned table causes a row to move to another partition, it will be performed as a `DELETE` from the original partition followed by an `INSERT` into the new partition. In this case, all row-level `BEFORE` `UPDATE` triggers and all row-level `BEFORE` `DELETE` triggers are fired on the original partition. Then all row-level `BEFORE` `INSERT` triggers are fired on the destination partition. The possibility of surprising outcomes should be considered when all these triggers affect the row being moved. As far as `AFTER ROW` triggers are concerned, `AFTER` `DELETE` and `AFTER` `INSERT` triggers are applied; but `AFTER` `UPDATE` triggers are not applied because the `UPDATE` has been converted to a `DELETE` and an `INSERT`. As far as statement-level triggers are concerned, none of the `DELETE` or `INSERT` triggers are fired, even if row movement occurs; only the `UPDATE` triggers defined on the target table used in the `UPDATE` statement will be fired.

No separate triggers are defined for `MERGE`. Instead, statement-level or row-level `UPDATE`, `DELETE`, and `INSERT` triggers are fired depending on (for statement-level triggers) what actions are specified in the `MERGE` query and (for row-level triggers) what actions are performed.

While running a `MERGE` command, statement-level `BEFORE` and `AFTER` triggers are fired for events specified in the actions of the `MERGE` command, irrespective of whether or not the action is ultimately performed. This is the same as an `UPDATE` statement that updates no rows, yet statement-level triggers are fired. The row-level triggers are fired only when a row is actually updated, inserted or deleted. So it's perfectly legal that while statement-level triggers are fired for certain types of action, no row-level triggers are fired for the same kind of action.

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 `NULL` to 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).

- For row-level `INSERT` and `UPDATE` triggers 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.

A row-level `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 `INSERT` and `UPDATE` triggers, the `OLD` row for `DELETE` triggers).

A row-level `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 `INSERT` and `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 `INSERT` and `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 `NULL`.

Some considerations apply for generated columns. Stored generated columns are computed after `BEFORE` triggers and before `AFTER` triggers. Therefore, the generated value can be inspected in `AFTER` triggers. In `BEFORE` triggers, the `OLD` row contains the old generated value, as one would expect, but the `NEW` row does not yet contain the new generated value and should not be accessed. In the C language interface, the content of the column is undefined at this point; a higher-level programming language should prevent access to a stored generated column in the `NEW` row in a `BEFORE` trigger. Changes to the value of a generated column in a `BEFORE` trigger are ignored and will be overwritten.

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 `BEFORE` and `INSTEAD OF` triggers, the possibly-modified row returned by each trigger becomes the input to the next trigger. If any `BEFORE` or `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 `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. `INSTEAD OF` triggers do not support `WHEN` conditions.

Typically, row-level `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` or `AFTER`, the `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 `UPDATE` trigger.

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., `INSERT` or `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 `INSERT` and `UPDATE` triggers, and/or the `OLD` row for `UPDATE` and `DELETE` triggers.

By default, statement-level triggers do not have any way to examine the individual row(s) modified by the statement. But an `AFTER STATEMENT` trigger can request that transition tables be created to make the sets of affected rows available to the trigger. `AFTER ROW` triggers can also request transition tables, so that they can see the total changes in the table as well as the change in the individual row they are currently being fired for. The method for examining the transition tables again depends on the programming language that is being used, but the typical approach is to make the transition tables act like read-only temporary tables that can be accessed by SQL commands issued within the trigger function.

Prev | Up | Next
Chapter 39. Triggers | Home | 39.2. Visibility of Data Changes