How to implement User-Defined Functions(UDF) in PostgreSQL?

User-Defined Functions (UDFs) in PostgreSQL allow you to define custom functions to extend the functionality of the database. UDFs can be written in various programming languages, such as SQL, PL/pgSQL, Python, or any other language supported by PostgreSQL. Let’s explore how to implement UDFs in PostgreSQL with a real-life data set example:

  1. Function Definition:

UDFs are created using the CREATE FUNCTION statement. The basic syntax for creating a UDF in PostgreSQL is as follows:

CREATE FUNCTION function_name ([argument1 data_type [, argument2 data_type, …]])
RETURNS return_type
LANGUAGE language_name
AS $$
— Function body goes here
$$

  • function_name: The name of the UDF.
  • argument1, argument2, …: Optional input arguments for the UDF, each with its corresponding data type.
  • return_type: The data type that the UDF returns.
  • language_name: The programming language used to write the UDF.
  • $$ … $$: The UDF body, which contains the code or SQL statements that define the UDF’s behavior.
  1. Example UDF:

Let’s consider a real-life example where we have a table named sales that stores information about sales transactions. We want to create a UDF that calculates the total sales amount for a given product ID. Here’s how you can implement the UDF in PostgreSQL:

CREATE FUNCTION calculate_total_sales(product_id integer)
RETURNS numeric
LANGUAGE plpgsql
AS $$
DECLARE
total_amount numeric;
BEGIN
SELECT SUM(sales_amount) INTO total_amount
FROM sales
WHERE product_id = calculate_total_sales.product_id;
RETURN total_amount;
END;
$$

In the above example, the UDF calculate_total_sales takes a single argument product_id of type integer and returns a numeric value. The UDF uses the plpgsql language, which is a procedural language similar to SQL. The function body calculates the sum of sales amounts for the given product_id from the sales table and returns the result.

  1. Calling the UDF:

Once the UDF is created, you can call it in SQL queries like any other built-in function. Here’s an example of calling the calculate_total_sales UDF:

SELECT product_id, calculate_total_sales(product_id) AS total_sales
FROM products;

In this query, we call the UDF calculate_total_sales for each row in the products table, passing the product_id column as the argument. The UDF calculates the total sales amount for each product and returns the result as the total_sales column.

UDFs in PostgreSQL provide a flexible way to extend the functionality of the database by creating custom functions. They can be used for a wide range of purposes, including data manipulation, complex calculations, data validation, and more. By writing UDFs, you can tailor PostgreSQL to suit your specific requirements and enhance the capabilities of your database.

About Shiv Iyer 390 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.