Postgres Triggers

Automatically execute SQL on table events.

In Postgres, a trigger executes a set of actions automatically on table events such as INSERTs, UPDATEs, DELETEs, or TRUNCATE operations.

Creating a trigger#

Creating triggers involve 2 parts:

1. A Function which will be executed (called the Trigger Function)
2. The actual Trigger object, with parameters around when the trigger should be run.

An example of a trigger is:

1create trigger "trigger_name"2after insert on "table_name"3for each row4execute function trigger_function();

Trigger functions#

A trigger function is a user-defined Function that Postgres executes when the trigger is fired.

Example trigger function#

Here is an example that updates salary_log whenever an employee's salary is updated:

1-- Example: Update salary_log when salary is updated2create function update_salary_log()3returns trigger4language plpgsql5as $$6begin7  insert into salary_log(employee_id, old_salary, new_salary)8  values (new.id, old.salary, new.salary);9  return new;10end;11$$;1213create trigger salary_update_trigger14after update on employees15for each row16execute function update_salary_log();

Trigger variables#

Trigger functions have access to several special variables that provide information about the context of the trigger event and the data being modified. In the example above you can see the values inserted into the salary log are old.salary and new.salary - in this case old specifies the previous values and new specifies the updated values.

Here are some of the key variables and options available within trigger functions:

• TG_NAME: The name of the trigger being fired.
• TG_WHEN: The timing of the trigger event (BEFORE or AFTER).
• TG_OP: The operation that triggered the event (INSERT, UPDATE, DELETE, or TRUNCATE).
• OLD: A record variable holding the old row's data in UPDATE and DELETE triggers.
• NEW: A record variable holding the new row's data in UPDATE and INSERT triggers.
• TG_LEVEL: The trigger level (ROW or STATEMENT), indicating whether the trigger is row-level or statement-level.
• TG_RELID: The object ID of the table on which the trigger is being fired.
• TG_TABLE_NAME: The name of the table on which the trigger is being fired.
• TG_TABLE_SCHEMA: The schema of the table on which the trigger is being fired.
• TG_ARGV: An array of string arguments provided when creating the trigger.
• TG_NARGS: The number of arguments in the TG_ARGV array.

Types of triggers#

There are two types of trigger, BEFORE and AFTER:

Trigger before changes are made#

Executes before the triggering event.

1create trigger before_insert_trigger2before insert on orders3for each row4execute function before_insert_function();

Trigger after changes are made#

Executes after the triggering event.

1create trigger after_delete_trigger2after delete on customers3for each row4execute function after_delete_function();

Execution frequency#

There are two options available for executing triggers:

• for each row: specifies that the trigger function should be executed once for each affected row.
• for each statement: the trigger is executed once for the entire operation (for example, once on insert). This can be more efficient than for each row when dealing with multiple rows affected by a single SQL statement, as they allow you to perform calculations or updates on groups of rows at once.

Dropping a trigger#

You can delete a trigger using the drop trigger command:

1drop trigger "trigger_name" on "table_name";

If your trigger is inside a restricted schema, you won't be able to drop it due to permission restrictions. In those cases, you can drop the function it depends on instead using the CASCADE clause to automatically remove all triggers that call it:

1drop function if exists restricted_schema.function_name() cascade;

Make sure you take a backup of the function before removing it in case you're planning to recreate it later.

Resources#

• Official Postgres Docs: Triggers
• Official Postgres Docs: Overview of Trigger Behavior
• Official Postgres Docs: CREATE TRIGGER