What is a PostgreSQL trigger?
Tags: apacheage, postgres, triggers
Published: 2023-07-14 10:04:05

A PostgreSQL trigger is a function that is called automatically whenever an event such as an insert, update, or deletion occurs on a table. Triggers can be used to perform a variety of tasks, such as:

* **Auditing:** Triggers can be used to track changes to data in a table. This can be useful for compliance purposes or for debugging.
* **Enforcing business rules:** Triggers can be used to enforce business rules on data. For example, a trigger could be used to ensure that a product price cannot be set below a certain value.
* **Generating data:** Triggers can be used to generate data automatically. For example, a trigger could be used to insert a new row into a table whenever a new user is created.

**Types of PostgreSQL triggers**

There are two types of PostgreSQL triggers: per-row triggers and per-statement triggers.

* **Per-row triggers** are invoked once for each row that is affected by the statement that fired the trigger. For example, if a trigger is defined on a table to fire when a row is updated, the trigger will be invoked once for each row that is updated.
* **Per-statement triggers** are invoked only once, regardless of how many rows are affected by the statement that fired the trigger. For example, if a trigger is defined on a table to fire when a row is deleted, the trigger will be invoked only once, even if multiple rows are deleted.

**Creating a PostgreSQL trigger**

To create a PostgreSQL trigger, you use the `CREATE TRIGGER` statement. The `CREATE TRIGGER` statement has the following syntax:

```
CREATE TRIGGER trigger_name
ON table_name
FOR [BEFORE | AFTER | INSTEAD OF] {UPDATE | INSERT | DELETE}
[WHEN condition]
EXECUTE function_name;
```

The `trigger_name` is the name of the trigger. The `table_name` is the name of the table on which the trigger is being created. The `FOR` clause specifies the type of trigger (BEFORE, AFTER, or INSTEAD OF). The `{UPDATE | INSERT | DELETE}` clause specifies the type of statement that will fire the trigger. The `WHEN condition` clause is optional and can be used to specify a condition that must be met in order for the trigger to be fired. The `function_name` is the name of the function that will be executed when the trigger is fired.

**Example of a PostgreSQL trigger**

The following is an example of a PostgreSQL trigger that is defined to fire after a row is updated:

```
CREATE TRIGGER update_timestamp
AFTER UPDATE
ON customers
FOR EACH ROW
EXECUTE function_update_timestamp();
```

This trigger will be invoked after each row in the `customers` table is updated. The `function_update_timestamp()` function will be executed to update the timestamp of the row.

**Managing PostgreSQL triggers**

Once you have created a PostgreSQL trigger, you can manage it using the following commands:

* `DROP TRIGGER`: This command is used to drop a trigger.
* `ALTER TRIGGER`: This command is used to modify a trigger.
* `ENABLE TRIGGER`: This command is used to enable a trigger.
* `DISABLE TRIGGER`: This command is used to disable a trigger.

**Conclusion**

PostgreSQL triggers are a powerful tool that can be used to automate tasks and enforce business rules. By understanding how to create and manage PostgreSQL triggers, you can improve the functionality and performance of your PostgreSQL databases.

I hope this article has been helpful. Please let me know if you have any questions.