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:
- A Function which will be executed (called the Trigger Function)
- The actual Trigger object, with parameters around when the trigger should be run.
An example of a trigger is:
_10create trigger "trigger_name"_10after insert on "table_name"_10for each row_10execute 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:
_16-- Example: Update salary_log when salary is updated_16create function update_salary_log()_16returns trigger_16language plpgsql_16as $$_16begin_16 insert into salary_log(employee_id, old_salary, new_salary)_16 values (new.id, old.salary, new.salary);_16 return new;_16end;_16$$;_16_16create trigger salary_update_trigger_16after update on employees_16for each row_16execute 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
orAFTER
).TG_OP
: The operation that triggered the event (INSERT
,UPDATE
,DELETE
, orTRUNCATE
).OLD
: A record variable holding the old row's data inUPDATE
andDELETE
triggers.NEW
: A record variable holding the new row's data inUPDATE
andINSERT
triggers.TG_LEVEL
: The trigger level (ROW
orSTATEMENT
), 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 theTG_ARGV
array.
Types of triggers
There are two types of trigger, BEFORE
and AFTER
:
Trigger before changes are made
Executes before the triggering event.
_10create trigger before_insert_trigger_10before insert on orders_10for each row_10execute function before_insert_function();
Trigger after changes are made
Executes after the triggering event.
_10create trigger after_delete_trigger_10after delete on customers_10for each row_10execute 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 thanfor 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:
_10drop trigger "trigger_name" on "table_name";
Resources
- Official Postgres Docs: Triggers
- Official Postgres Docs: Overview of Trigger Behavior
- Official Postgres Docs: CREATE TRIGGER