DbSchema | How to Create a Trigger in PostgreSQL?
Table of Contents
- Introduction
- What is a Trigger?
- Advantages and Limitations of Using a Trigger
- Restrictions on Creating a Trigger
- Types of Triggers and Their Applicability
- Parameters for Creating a Trigger
- Step-by-Step Guide to Creating a Trigger in psql and DbSchema
- Conclusion
- References
Introduction
PostgreSQL
is a powerful open-source relational database management system that provides extensive support for creating triggers
. Triggers are database objects that allow you to define custom actions that are automatically executed in response to specified events on a table. This article will explain what triggers are, their advantages and limitations, the restrictions on creating them, and provide a step-by-step guide on how to create triggers using psql
and DbSchema
.
What is a Trigger?
A trigger
in PostgreSQL is a set of SQL statements that are automatically executed when a specific event occurs on a table. These events can include INSERT
, UPDATE
, or DELETE
operations. Triggers can be defined to act either before or after the event, or instead of the event. They can be applied to tables, views, or foreign tables.
Advantages and Limitations of Using a Trigger
Advantages:
Triggers offer several advantages in database management:
- Automation: Triggers allow you to
automate
certain actions without requiring manual intervention, ensuring data consistency and integrity. - Data Validation: Triggers can enforce
data validation rules
, preventing invalid or inconsistent data from being inserted or updated in the database. - Complex Logic: Triggers enable the execution of
complex business logic
that cannot be easily achieved through standard SQL operations alone.
Limitations:
However, it is important to consider the limitations of triggers:
- Performance Impact: Poorly designed triggers or triggers with extensive logic can negatively impact
database performance
, so careful optimization is required. - Maintenance Complexity: Triggers introduce additional
complexity
to the database schema, making it harder to understand and maintain.
Restrictions on Creating a Trigger
While PostgreSQL provides great flexibility in defining triggers, there are certain restrictions to keep in mind:
- Triggers cannot be defined on
system catalogs
ortemporary tables
. - For table-level triggers, the table must be
specified
in the same schema as the trigger. - Triggers cannot be created on views that include
aggregates
orwindow functions
. - Certain statements, such as
TRUNCATE
orDROP DATABASE
, cannot be used within triggers.
Types of Triggers and Their Applicability
Triggers
in PostgreSQL can be categorized based on their timing (when they are executed) and their granularity (row-level or statement-level).
The following table summarizes
the types of triggers that can be used on tables, views, or foreign tables:
When | Event | Row-level Trigger | Statement-level Trigger |
---|---|---|---|
BEFORE |
INSERT/UPDATE/DELETE | Tables and foreign tables | Tables, views, and foreign tables |
TRUNCATE | — | Tables | |
AFTER |
INSERT/UPDATE/DELETE | Tables and foreign tables | Tables, views, and foreign tables |
TRUNCATE | — | Tables | |
INSTEAD OF |
INSERT/UPDATE/DELETE | Views | — |
TRUNCATE | — | — |
Parameters for Creating a Trigger
When creating a trigger, you need to specify several parameters
. Here is a summary of the most important parameters:
Parameter | Description |
---|---|
Name |
The name of the trigger. |
Event |
The event that triggers the execution of the trigger (INSERT, UPDATE, or DELETE). |
Table_name |
The name of the table on which the trigger is created. |
Referenced_table_name |
The name of the referenced table, if the trigger is a foreign key trigger. |
REFERENCING |
The transition relation name for row-level triggers. |
Transition_relation_name |
The name of the table or view that the trigger is associated with. |
Condition |
An optional condition that must be satisfied for the trigger to execute. |
Function_name |
The name of the function to be executed when the trigger fires. |
Arguments |
Additional arguments to be passed to the trigger function. |
Step-by-Step Guide to Creating a Trigger in psql and DbSchema
Creating a Trigger in psql:
- Open a
psql
session and connect to your PostgreSQL database.
- Open a
For installation and establishing connection refer to PostgreSQL-How to create a database?
- Make sure you have created a
table
in your database previously.
For creating a table refer to PostgreSQL-How to create a table?
Execute
the following command to create a trigger:
Replace the placeholders with the appropriate values for your trigger.CREATE TRIGGER trigger_name {BEFORE | AFTER | INSTEAD OF} {INSERT | UPDATE | DELETE} ON table_name [REFERENCING {OLD | NEW} TABLE transition_relation_name] [FOR EACH {ROW | STATEMENT}] [WHEN (condition)] EXECUTE FUNCTION function_name(arguments);
Example:
Let’s consider an example
where we want to create a trigger named “update_salary_trigger” that automatically updates the “total_salary” column in the “employees” table whenever an update operation is performed on the “salary” column.
CREATE TRIGGER update_salary_trigger
BEFORE UPDATE
ON employees
FOR EACH ROW
WHEN (OLD.salary <> NEW.salary)
EXECUTE FUNCTION update_total_salary();
Creating a Trigger in DbSchema:
- Open
DbSchema
and connect to your PostgreSQL database. Navigate
to the “Triggers” section and click on “Add Trigger”.Fill in
the required parameters such as name, event, table name, etc.Specify
the trigger function and any additional arguments.- If needed,
define
the referencing table and choose the row-level or statement-level execution. Add a condition
if necessary.Save
the trigger andapply the changes
to the database.
- Open
Example:
- Navigate to the “Triggers” section and click on “Add Trigger”.
- Fill in the parameters as follows:
- Name: update_salary_trigger
- Event: BEFORE UPDATE
- Table_name: employees
- Transition_relation_name: OLD and NEW
- Referencing_table_name: leave blank
- Row-level: checked
- Condition: OLD.salary <> NEW.salary
- Function_name: update_total_salary
- Arguments: leave blank or provide any required arguments
- Fill in the parameters as follows:
Create Triggers and Visually Manage PostgreSQL using DbSchema
DbSchema is a PostgreSQL
client and visual designer
. DbSchema has a free Community Edition, which can be downloaded here.
Create Trigger
- Start the application and connect to the Postgres database.
- Navigate to
Triggers
section and click onAdd Trigger
. - Fill in required parameters and save the trigger.
Conclusion
Triggers
in PostgreSQL provide a powerful mechanism for automating actions and enforcing data integrity. By understanding what triggers are, their advantages and limitations, the restrictions on creating them, and following the step-by-step guide, you can effectively incorporate triggers into your database management workflow.
References
- PostgreSQL Official Documentation: https://www.postgresql.org/docs/
- DbSchema Documentation: https://www.dbschema.com/documentation/
- SQL Syntax: https://www.w3schools.com/sql/
- PostgreSQL Tutorial: https://www.postgresqltutorial.com/
- DbSchema Interactive Diagrams: https://www.dbschema.com
Remember, the official documentation for both PostgreSQL and DbSchema is the most reliable source for up-to-date information. These resources can provide more in-depth knowledge and cover other complex aspects of creating and managing databases.