PostgreSQL Triggers: Simplifying Automated Actions

PostgreSQL triggers offer a powerful mechanism for automating actions within the database.

Trigger Basics

• Triggers execute specific functions automatically upon defined operations.
• They can be attached to tables, views, and foreign tables.
• Triggers can fire before, after, or instead of INSERT, UPDATE, DELETE, and TRUNCATE operations.

Varieties of Triggers

1. Row-Level Triggers: Executed once per modified row.
2. Statement-Level Triggers: Executed once per SQL statement.
3. INSTEAD OF Triggers: Used on views to modify underlying tables.
4. BEFORE Triggers: Fired before the operation.
5. AFTER Triggers: Fired after the operation.
6. INSTEAD OF Triggers: Fired instead of operation (views only).

Trigger Execution Sequence

• Triggers are invoked in alphabetical order by trigger name.
• If a BEFORE trigger returns NULL, subsequent triggers are not fired for that row.
• Statement-level triggers are executed even if no rows are affected.

Trigger Functions and Cascading

• A trigger function is defined before creating the trigger.
• Trigger functions are invoked within the same transaction as the statement.
• Be cautious of cascading triggers that might lead to recursion.

Examining Trigger Data

• Trigger input data includes the type of event (e.g., INSERT, UPDATE) and arguments.
• Row-level triggers have access to NEW (INSERT/UPDATE) and OLD (UPDATE/DELETE) rows.
• Statement-level triggers can request transition tables to access affected rows.

Practical Uses of Triggers

• Validation: Enforce data consistency with checks before modification.
• Auditing: Log changes for auditing purposes using AFTER triggers.
• Synchronization: Propagate updates to related tables.
• Default Values: Automatically insert default values.
• Data Transformation: Modify data based on specific criteria.
• View Modifications: Modify views with INSTEAD OF triggers.

Conclusion

PostgreSQL triggers simplify and streamline tasks by automating specific actions in response to data modifications. By grasping the distinctions between row-level, statement-level, and INSTEAD OF triggers, developers can harness their power to enhance data integrity, tracking, and overall database functionality.

Reference

triggers