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Understanding Triggers In PostgreSQL

In the dynamic world of relational databases, where precision and efficiency are paramount, PostgreSQL emerges as a robust solution. At the heart of its functionality lies a feature that adds a layer of intelligence and automation to database operations – Triggers.

What exactly are triggers? They are predefined actions that the database executes automatically when a specific event occurs, such as an insertion, update, or deletion of records in a table. Think of triggers as the silent guardians behind the scenes, ensuring data integrity, enforcing business rules, and automating repetitive tasks.

Imagine you have an online store, and every time a new product is added, you want to update its timestamp automatically. Instead of manually managing this, a trigger can be set up to do it for you. So, when a new product is added, the trigger instantly updates the timestamp effortlessly. That’s the magic of triggers – they automate actions in response to specific events, making your database smarter and more efficient.

Benefits of using triggers

Maintain Data Consistency – Triggers help maintain data consistency by automatically enforcing rules and validations, reducing the risk of inconsistent or invalid data.

Automation of Routine Tasks – They automate routine tasks, such as updating timestamps, generating audit logs, or implementing complex default values, reducing the burden on application code.

Effective Logging and Auditing – Triggers facilitate the creation of detailed logs and audit trails, crucial for compliance, troubleshooting, and understanding the history of data changes.

Security Measures – Triggers can be employed to implement security measures, such as restricting access or masking sensitive data based on certain conditions.

Automated Timestamps – Triggers can automatically update timestamp fields, ensuring that records reflect the latest modification time without relying on application code.

Enforcing Data Integrity – Enforcing Data InteTriggers can be used to enforce rules and constraints on the data, ensuring that it meets specific criteria before being inserted or updated. For example, preventing the insertion of negative values or ensuring referential integrity.grity:

BEFORE vs AFTER Triggers

BEFORE triggers are fired before the execution of the associated event (e.g., before an INSERT, UPDATE, DELETE, or TRUNCATE operation). They are commonly used to validate or modify data before it is actually written to the table. If a BEFORE trigger returns NULL or an empty result set, the original operation (e.g., INSERT, UPDATE) is canceled.
AFTER triggers are fired after the execution of the associated event. They are useful for tasks that should occur after the data has been modified in the table, such as logging changes or updating other related tables.

Row-level Triggers

Row-level triggers in PostgreSQL are a specific type of database trigger designed to run separately for each affected row. These triggers are linked to a particular table and are triggered by events like INSERT, UPDATE, DELETE, or TRUNCATE. They prove valuable when you want to execute actions or checks specific to each row that undergoes modification.

Statement-level Triggers

A statement-level trigger in a database, often referred to as a “summary” trigger, operates on the entirety of a given SQL statement. This type of trigger executes once for the entire statement, offering a mechanism to carry out actions based on the collective outcome of the operation, as opposed to focusing on individual rows.

When a statement-level trigger is employed, it responds to specific SQL statements such as INSERT, UPDATE, DELETE, or TRUNCATE. Unlike row-level triggers, which act on each affected row individually, statement-level triggers consider the overarching impact of the entire operation on the dataset.

More from the Blog:
Triggers in PostgreSQL: Love them or hate them?


INSERT event triggers

When a new record is introduced to your database, the INSERT event trigger comes into play. Imagine an e-commerce platform where a new product is added to the inventory. With an INSERT trigger, you can effortlessly execute actions such as updating timestamps, initializing default values, or even triggering notifications – all seamlessly integrated into the process of adding a new entity.

example to set up an AFTER INSERT trigger in PostgreSQL that logs employee insertions by automatically recording details, such as employee ID and timestamp, into an employee_log table.

Create employees table

CREATE TABLE employees (
    employee_id SERIAL PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    salary NUMERIC

Create employee_log table

CREATE TABLE employee_log (
    employee_id INT,
    action_type VARCHAR(10)

Now, create the trigger function

CREATE OR REPLACE FUNCTION log_employee_insert()
    INSERT INTO employee_log (employee_id, action_type)
    VALUES (NEW.employee_id, 'INSERT');
$$ LANGUAGE plpgsql;

The above provided PostgreSQL code defines a trigger function named log_employee_insert. This function is designed to execute automatically after an INSERT operation on a table. When a new record is inserted into a table, it inserts corresponding details, such as the employee_id and the action type (‘INSERT‘), into an employee_log table. The function uses the NEW keyword to access the values of the newly inserted row and then returns the NEW row. This trigger function is written in PL/pgSQL, which is PostgreSQL’s procedural language.

Finally, create the trigger

CREATE TRIGGER after_employee_insert
EXECUTE FUNCTION log_employee_insert();

The above PostgreSQL code creates an AFTER INSERT trigger named after_employee_insert on the employees table. This trigger is set to execute the log_employee_insert function for each inserted row. When a new record is inserted into the employees table, the trigger automatically invokes the log_employee_insert function, which logs relevant information into the employee_log table.

Now, whenever you insert a new record into the employees table, the trigger will automatically log the event in the employee_log table.

INSERT INTO employees (first_name, last_name, salary) VALUES ('John', 'Doe', 50000);

Select employees table

select * from employees;
 employee_id | first_name | last_name | salary 
           1 | John       | Doe       |  50000
(1 row)

Select employee_log table

select * from employee_log;
 log_id |     action_timestamp      | employee_id | action_type 
      1 | 2023-12-14 10:17:58.53535 |           1 | INSERT
(1 row)

UPDATE event triggers

When existing records undergo modifications, the UPDATE event trigger takes charge. Think of a user profile that needs to be updated with the latest information. Utilizing an UPDATE trigger, you can ensure that associated data, logs, or calculations are promptly adjusted, maintaining the coherence and accuracy of your database.volution of Data

Create extension hstore it is a key-value store that enables the storage and retrieval of sets of key-value pairs within a single database column so we can save data like “first_name”=>”semab” in a single PostgreSQL column


Update the existing employee_log table to add a new column so we can track what exactly has been updated in the original table

ALTER TABLE employee_log ADD COLUMN updated_fields HSTORE;

Create trigger function

CREATE OR REPLACE FUNCTION log_employee_update()
    INSERT INTO employee_log (employee_id, action_type, updated_fields)
    VALUES (NEW.employee_id, 'UPDATE', hstore(NEW.*) - hstore(OLD.*));
$$ LANGUAGE plpgsql;

The above PostgreSQL code defines a trigger function named log_employee_update that automatically executes after an UPDATE operation on a specified table. The function logs details of the update and the specific fields that were modified, along with their new values, into the employee_log table. The hstore(NEW.*) – hstore(OLD.*) expression calculates the difference between the new and old row values, capturing updated fields. The function then returns the updated row.

Create the trigger

CREATE TRIGGER after_employee_update
EXECUTE FUNCTION log_employee_update();

Update the record in the original table

UPDATE employees SET first_name = 'semab' WHERE employee_id = 1;

Select the employee_log table

select * from employee_log;
 log_id |      action_timestamp      | employee_id | action_type |    updated_fields     
      1 | 2023-12-14 10:17:58.53535  |           1 | INSERT      | 
      2 | 2023-12-14 10:33:28.442459 |           1 | UPDATE      | "first_name"=>"semab"
(2 rows)

DELETE event triggers

Deleting records demands careful consideration, and the DELETE event trigger ensures a graceful exit. Picture a scenario where a customer unsubscribes from a service. With a DELETE trigger, you can not only remove the customer data but also initiate cascading actions, such as updating analytics or sending a farewell email – streamlining the process of gracefully parting ways with data.

Create the trigger function

CREATE OR REPLACE FUNCTION log_employee_delete()
    INSERT INTO employee_log (employee_id, action_type)
    VALUES (OLD.employee_id, 'DELETE');
$$ LANGUAGE plpgsql;

Create the trigger

CREATE TRIGGER after_employee_delete
EXECUTE FUNCTION log_employee_delete();

Delete the record

DELETE FROM employees WHERE employee_id = 1;

Select employee_log table

select * from employee_log;
 log_id |      action_timestamp      | employee_id | action_type |    updated_fields     
      1 | 2023-12-14 10:17:58.53535  |           1 | INSERT      | 
      2 | 2023-12-14 10:33:28.442459 |           1 | UPDATE      | "first_name"=>"semab"
      3 | 2023-12-14 10:41:06.659958 |           1 | DELETE      | 
(3 rows)

How to rename a trigger

There are situations where you might want to rename a trigger. This could be due to changes in naming conventions, improving clarity, or simply correcting a mistake in the trigger’s name. Renaming triggers is a common administrative task that ensures your database remains well-organized and maintainable.

ALTER TRIGGER old_trigger_name ON table_name RENAME TO new_trigger_name;

How to drop a trigger

To drop (remove) a trigger in PostgreSQL, you use the DROP TRIGGER command. 

DROP TRIGGER [IF EXISTS] [schema_name.]trigger_name ON table_name [CASCADE | RESTRICT];

IF EXISTS – This is optional and prevents an error from occurring if the trigger doesn’t exist.

schema_name – The name of the schema where the trigger is located. This is optional, and if not specified, PostgreSQL assumes the trigger is in the public schema.

trigger_name – The name of the trigger you want to drop.

table_name – The name of the table on which the trigger is defined.

CASCADE | RESTRICT – These are also optional. If CASCADE is specified, it automatically drops objects that depend on the trigger (like rules or other triggers). If RESTRICT is specified, the trigger will only be dropped if there are no dependent objects.

Best Practices to Use Triggers

Keep Triggers Simple – Aim to keep trigger logic simple and focused on specific tasks. Complex triggers can be difficult to maintain and may lead to unexpected behavior.

Document Trigger Logic – Document the purpose and logic of your triggers thoroughly. This documentation helps other developers understand the intent behind the trigger and facilitates future maintenance.

Test Thoroughly – Thoroughly test triggers in a development environment before deploying them to production. Consider edge cases and potential interactions with other parts of the database schema.

Consider Performance Impact – Understand the performance implications of triggers, especially in high-transaction environments. Use appropriate indexes and optimize trigger logic to minimize any negative impact on database performance.

Regularly Review and Refactor – Periodically review triggers as part of ongoing database maintenance. Refactor triggers when necessary to accommodate changes in business requirements or to improve performance.

Consider Alternative Approaches – Before implementing triggers, consider alternative approaches, such as using stored procedures or application-level logic. Evaluate which method aligns best with your overall system architecture and design principles.

Be Mindful of Transaction Isolation – Understand the transaction isolation level in PostgreSQL. Triggers operate within the context of the transaction, and changes made by triggers are subject to transaction isolation rules.

Avoid Excessive Logging – Limit the amount of logging or debugging information generated by triggers, especially in a production environment. Excessive logging can impact performance, so use it judiciously and consider conditional logging based on environment or configuration settings.

As we conclude this exploration of trigger management in PostgreSQL, remember that a well-organized and thoughtfully designed database is a cornerstone of robust application development. Whether you’re fine-tuning triggers for better clarity or removing them as project requirements evolve, each action contributes to the overall health and longevity of your database.

Comments (2)

  • Slava

    December 19, 2023 - 5:40 am

    Great summary of how triggers can and should be used!
    I particularly like the trick using hstore to log the changes to the fields (hstore(NEW.*) – hstore(OLD.*)).
    One question: is there a way to use hstore to log the change to values in addition to which column changed. Eg: ‘first_name: “John” => “semab” ‘. This would be great for CDC when updating target tables in Analytics.

    • Semab Tariq

      December 19, 2023 - 12:30 pm

      Hello Slava,

      Thank you for your suggestion! It’s an excellent idea to capture the values that are about to be updated. To implement this, I propose creating a new column named old_values within the employee_log table. Here’s the updated table structure:

      CREATE TABLE employee_log (
      log_id SERIAL PRIMARY KEY,
      employee_id INT,
      action_type VARCHAR(10),
      updated_fields HSTORE,
      old_values HSTORE

      Additionally, the stored procedure has been modified to capture only the values that are being updated:

      CREATE OR REPLACE FUNCTION log_employee_update()
      INSERT INTO employee_log (employee_id, action_type, updated_fields, old_values)
      VALUES (
      hstore(NEW.*) - hstore(OLD.*),
      hstore(OLD.*) - hstore(NEW.*)
      $$ LANGUAGE plpgsql;

      This enhancement ensures that the old_values column captures only the fields that have been modified during an update operation. i.e,

      postgres=# select * from employee_log;
      log_id | action_timestamp | employee_id | action_type | updated_fields | old_values
      1 | 2023-12-19 12:09:09.248974 | 1 | INSERT | |
      2 | 2023-12-19 12:09:39.840477 | 1 | UPDATE | "first_name"=>"semab" | "first_name"=>"John"
      (2 rows)

      Best regards,
      Semab Tariq

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