What does the create trigger statement in PostgreSQL do?

What does the create trigger statement in PostgreSQL do?

PostgreSQL also provides other local variables preceded by TG_ such as TG_WHEN, and TG_TABLE_NAME. Once you define a trigger function, you can bind it to one or more trigger events such as INSERT, UPDATE, and DELETE. The CREATE TRIGGER statement creates a new trigger.

When to return NULL in PostgreSQL trigger function?

Row-level triggers fired BEFORE can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the INSERT / UPDATE / DELETE does not occur for this row). If a nonnull value is returned then the operation proceeds with that row value.

How to refer to newly inserted values in trigger function?

Within the trigger function you can use NEW.column_name to refer to the newly inserted / updated value. In the reverse, OLD.column_name will refer to the value prior to the update / delete. Hope that helps. Within the trigger function you can use NEW.column_name to refer to the newly inserted or updated value.

When to return the row value of a trigger function?

Returning a row value different from the original value of NEW alters the row that will be inserted or updated. Thus, if the trigger function wants the triggering action to succeed normally without altering the row value, NEW (or a value equal thereto) has to be returned.

What does old and new mean in PostgreSQL?

The OLD represents the row before update while the NEW represents the new row that will be updated. The OLD.last_name returns the last name before the update and the NEW.last_name returns the new last name. Second, bind the trigger function to the employees table.

When to use old and New in MySQL triggers?

In a UPDATE trigger, you can use OLD.col_name to refer to the columns of a row before it is updated and NEW.col_name to refer to the columns of the row after it is updated. In a DELETE trigger, only OLD.col_name can be used; there is no new row. A column named with OLD is read only.

Do you need to pass parameters to the trigger function?

You do not need to pass the NEW and OLD as parameters to the trigger function. They are automagically available there: http://www.postgresql.org/docs/9.1/interactive/trigger-definition.html : The trigger function must be declared as a function taking no arguments and returning type trigger.

How to declare a trigger function in SQL?

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.)

When to fire constraint trigger in Postgresql 9.0?

Constraint triggers must be AFTER ROW triggers. 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.

Why are trigger functions opaque in PostgreSQL Version 7.3?

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 PostgreSQL versions before 7.3, it was necessary to declare trigger functions as returning the placeholder type opaque, rather than trigger.

What does when condition refer to in PostgreSQL?

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.

When to use a PostgreSQL trigger consistency check?

Employees have a salary column, and the sum of salaries in a given department should not exceed the budget column value of the associated department table record. PostgreSQL supports the SQL standard CHECK constraints, and we used them for the SINGLE_TABLE JPA inheritance strategy.