PostgreSQL Trigger

In this section, we are going to understand the working of the PostgreSQL Triggers, why we need to use the triggers and when to use them and also see the merits and demerits of PostgreSQL triggers, features of PostgreSQL Triggers and various command, which are performed under the PostgreSQL Trigger section.

What are PostgreSQL Triggers?

A trigger is a special user-defined function connected with a table. if we want to generate a new trigger:

• Firstly, we can specify a trigger function.
• Secondly, bind the same trigger function to a table.

Note: The major variance between a trigger and a user-defined function is that, when any triggering event occurs, a trigger is automatically raised.

A PostgreSQL Trigger is a function, which involved automatically whenever an event linked with a table. The event can be described as any of the following INSERT, UPDATE, DELETE or TRUNCATE.

Type of Triggers

In PostgreSQL, the trigger can be categorized into two parts, which are as follows:

• Row-level trigger
• Statement-level trigger

For example, if we issue an UPDATE command, which affects 10 rows, the row-level trigger will be invoked 10 times, on the other hand, the statement level trigger will be invoked 1 time.

How can Trigger be used in PostgreSQL?

• A trigger can also be marked with the FOR EACH command operator through its creation, and the same trigger can be implemented only once for a particular operation.
• We can use the trigger with the FOR EACH ROW operator in its creation, and those triggers will be called once for each row changed by the operation.

Usage of Triggers

The triggers can be used in the following aspects:

• The triggers can be used to authenticate the input data.
• The triggers can also implement business rules.
• It can easily retrieve the system functions.
• The triggers can be used to create a unique value for a newly-inserted row in a diverse file.
• By using trigger, we can repeat the data in a different file to reach data reliability.
• It is used to write to add files to fulfil the objective of the audit trail.
• The trigger can be used to get the data from other files for cross-referencing objectives.

The Various command used in PostgreSQL Triggers

In PostgreSQL trigger, we can perform the following commands:

• CREATE Trigger
• ALTER Trigger
• DROP Trigger
• ENABLE Trigger
• DISABLE Trigger

Let us understand them one by one:

• Create trigger: In PostgreSQL, the CREATE TRIGGER command generates our first trigger step by step.
• Alter trigger: The ALTER TRIGGER command is used to rename a trigger.
• Drop trigger: The DROP TRIGGER command is used to define the steps to remove a trigger from a table
• Enable triggers: In the PostgreSQL trigger, the ENABLE TRIGGER statement allows a trigger or all triggers related to a table.
• Disable trigger: The DISABLE TRIGGER is used to display how we can disable a trigger or all triggers linked with a table.

Features of PostgreSQL Triggers

Some of the essential features of the PostgreSQL triggers are as follows:

• PostgreSQL will execute the trigger for the TRUNCATE event.
• PostgreSQL allows us to specify the statement-level trigger on views.
• PostgreSQL needs to specify a user-defined function as the action of the trigger, whereas the SQL standard provides us to use any SQL commands.

The Advantage of using PostgreSQL Trigger

We have the following advantages of using the PostgreSQL trigger:

• The PostgreSQL trigger is used to enhance the client-server environment's performance as all guidelines are executed on the server before the output retrieves.
• PostgreSQL Triggers are beneficial if several applications retrieve the database. We need to retain the cross-functionality within the database, which is executed repeatedly whenever the table's data is changed.
  • For instance, if we want to keep the past of data without needing the application to have reason for verifying each event such as UPDATE or INSERT.
• It will accelerate the application development as the database stores triggers, and we do not have to write the code for trigger actions into each database application.
• We can define whether the trigger is requested before or after an event:
  • Suppose the trigger is requested after the event, then all modification is accessible to the trigger.
  • Or if the trigger is requested before an event, it can be modified the row being inserted or updated, or we can skip the operation for the existing row.
• The importance of using the PostgreSQL trigger, we can easily monitor the business policy alteration, and we only need to modify the equivalent trigger program in its place of every application program.
• We can also use triggers to contain the complicated data integrity rules, which we cannot execute in another place, excluding at the database level.
  • Such as, when a new row is inserted to the Employeetable, other rows must also be generated in Department and Jobs

The Drawback of using the PostgreSQL Trigger

The major drawback of using a PostgreSQL trigger is that we must know the trigger occurs and understand its logic and the effects when data is modified.

Overview

In the PostgreSQL triggers section, we have learned the following topics:

• We have learned about the PostgreSQL Triggers.
• We have understood the usages of triggers and how we can use the triggers in PostgreSQL.
• We have understood the features of PostgreSQL Trigger.
• Brief introduction of Various PostgreSQL Commands such as CREATE, ALTER, DROP, ENABLE, DISABLE.
• Advantages and disadvantages of using PostgreSQL Trigger.