Postgres Triggers: Best Practices and Common Use Cases – PGSQL Phriday #007

PGSQL Phriday #007 – Be the James Bond of your Database – take risks, Create Triggers, and always remember that you have a license to thrill.

Lætitia Avrot is hosting the #PGSQLPhriday 007
PGSQL Phriday is a monthly community blog event by the PostgreSQL community.

Postgres Triggers is a powerful tool for automatically executing custom code when certain events occur in the database, such as INSERTs, UPDATEs, or DELETEs in a table. While Triggers can be very useful for enforcing business rules, auditing changes to data, and maintaining data consistency, they can potentially corrupt the database if they are not designed and implemented carefully.

Some popular Trigger pitfalls:

• Endless loops: Triggers can cause infinite loops if they update the same table or tables that are referenced by other Triggers. This can lead to excessive CPU usage and lock the database, resulting in poor performance and even crashing the database.
• Conflicts: Triggers can conflict with each other if they are designed to perform the same action on the same table or column. This can result in unexpected behavior, data inconsistencies, and even data loss.
• Errors: Triggers can generate errors if they contain bugs or are not designed to handle certain edge cases. This can cause data corruption, inconsistent data, or even crashing the database.

Some best practices to follow:

• Use sparingly: Triggers should be used only when necessary, and only on the tables where they are needed. Too many triggers can slow down the database and cause unnecessary complexity.
• Efficient code: Triggers should be designed to execute quickly and efficiently. This can include using appropriate indexes, optimizing queries, and minimizing the use of unnecessary database resources.
• Test thoroughly: Triggers should be thoroughly tested before they are deployed to the production environment. This can include testing for conflicts, performance issues, and error handling.
• Monitoring: Triggers should be monitored regularly to ensure that they are executing as expected and that they are not causing excessive load on the database.

Now that we’ve looked at a bit of Pros & Cons, let’s look at Postgres triggers that can be used for two main purposes:

• Business logic flow
• Audit flow

Business logic flow: To enforce business rules by automatically executing custom code when certain events occur in the database. For example, a trigger can be defined to prevent certain rows from being inserted into a table or to update the values of certain columns based on the values in other columns. This can help to ensure data consistency and enforce business rules. 

Audit flow: To audit changes to data by automatically recording information about changes that are made to a table. For example, a trigger can be defined to record the user who made a change, the time when the change was made, and the old and new values of the changed data. This can help to track changes to data over time and identify potential issues.

Both business logic and audit flows can be useful in different contexts, depending on the specific requirements of the application. It’s important to design and implement triggers carefully, however, to ensure that they execute efficiently and do not cause performance issues or data corruption.

Event triggers

Event triggers in Postgres are specifically used to execute custom code in response to specific events that occur in the database. Unlike regular triggers, which are associated with specific tables and execute when changes are made to those tables, event triggers are associated with specific events that occur at the database level, such as the creation or deletion of a table, or the start or end of a transaction.

Defined using the `CREATE EVENT TRIGGER` statement, and can be used for a variety of purposes, including:

Schema management: Event triggers can be used to automate schema management tasks, such as creating or modifying tables, indexes, or other database objects in response to specific events.
Security management: Event triggers can be used to enforce security policies.
Maintenance: Can be used to automate maintenance tasks, such as archiving data to reduce disk space usage.

Conclusion
Postgres triggers provide a mechanism for responding to changes in your database by executing custom code in response to specific events. Triggers can be used to enforce business rules, validate data, automate tasks, and perform audit logging. They are highly customizable and can be created at the row, statement, or database level, allowing you to tailor their behavior to your specific needs. While triggers can be a powerful tool, they should be used judiciously to avoid negatively impacting database performance.