Validating Inserts with PostgreSQL Triggers and User-Defined Functions

PostgreSQL provides several ways to validate data before insertion, including triggers and user-defined functions (UDFs). In this article, we will explore how to use both methods to check if a tuple satisfies a specific condition before inserting it into a table.

Introduction

When working with databases, it’s essential to ensure that the data being inserted meets certain criteria. This can be done using various validation techniques, including triggers and UDFs. In this article, we’ll focus on two popular methods: creating a trigger that checks for a specific condition before inserting a tuple into a table.

Triggers

A trigger is a stored procedure that’s automatically executed when a specific event occurs, such as an insertion or update to a row. In our case, we want to create a trigger that checks if the tuple being inserted into table a already exists in table b.

Example Trigger Code

CREATE OR REPLACE FUNCTION myfunc() 
RETURNS TRIGGER AS $BODY$
BEGIN
 IF EXISTS (SELECT 1 FROM b WHERE b.y =NEW.y AND b.x=NEW.x ) THEN
   RAISE EXCEPTION 'tuple already exists in "b": % %', NEW.x,NEW.y;
 END IF;
 RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql;

CREATE TRIGGER check_tupel
BEFORE INSERT OR UPDATE ON a
FOR EACH ROW EXECUTE PROCEDURE myfunc();

In the above code:

We create a UDF called myfunc() that takes no arguments and returns TRIGGER.
Inside the function, we use a SELECT statement to check if the tuple being inserted into table a already exists in table b.
If the tuple exists, we raise an exception using RAISE EXCEPTION.
Otherwise, we return the original row using RETURN NEW;.

Checking for Tuple Existence

To check if a tuple exists in another table, we can use a simple SELECT statement with the EXISTS clause. However, this method requires us to fetch all columns of both tables and compare them.

Alternative Solution

Instead of creating a trigger, you could create a CHECK constraint using a user-defined function (UDF). This approach allows you to validate data without affecting the performance of inserts.

User-Defined Functions

A UDF is a custom stored procedure that can be used with SQL commands. In our case, we want to create a UDF that checks if a tuple exists in table b.

Example UDF Code

CREATE OR REPLACE FUNCTION myfunc(x int, y int) 
RETURNS BOOLEAN AS $BODY$
 SELECT EXISTS (SELECT 1 FROM b WHERE b.y =$1 AND b.x=$2 )
$BODY$
LANGUAGE sql;

In the above code:

We create a UDF called myfunc() that takes two integer arguments (x and y) and returns a boolean value.
Inside the function, we use a SELECT statement to check if the tuple exists in table b.

Creating a CHECK Constraint

To create a CHECK constraint using the UDF, we need to define a CHECK constraint that calls the UDF.

Example CHECK Constraint Code

CREATE TABLE a (
  x int,
  y int,
 CONSTRAINT exists_in_b CHECK (myfunc(x,y))
);

In the above code:

We create a table a with two integer columns (x and y) and a CHECK constraint called exists_in_b.
The CHECK constraint references our UDF myfunc() and calls it with the values of x and y.

Using the CHECK Constraint

When we try to insert data into table a, PostgreSQL will execute the CHECK constraint. If the tuple does not satisfy the condition, an exception will be raised.

Example Insert Statement

INSERT INTO a VALUES (42,42);

In this case, since the tuple (42, 42) already exists in table b, an exception will be raised:

Error Message

new row for relation "a" violates check constraint "exists_in_b"

Conclusion

PostgreSQL provides two powerful methods for validating data before insertion: triggers and user-defined functions. In this article, we explored how to use both methods to check if a tuple satisfies a specific condition. By using a trigger or a UDF, you can enforce data integrity and prevent invalid data from being inserted into your tables.

Recommendations

Use triggers when you need to perform complex validation logic that involves multiple tables or processes.
Use UDFs when you want to create reusable validation functions that can be applied across multiple tables.
Always consider performance implications when using triggers or UDFs, as they may impact the speed of inserts and updates.