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
- Row-Level Triggers: Executed once per modified row.
- Statement-Level Triggers: Executed once per SQL statement.
- INSTEAD OF Triggers: Used on views to modify underlying tables.
- BEFORE Triggers: Fired before the operation.
- AFTER Triggers: Fired after the operation.
- 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.
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