User-defined functions (UDFs) are a key feature of PostgreSQL, a popular and powerful open-source relational database management system. UDFs allow you to create functions according to your specific requirements and extend the functionality of PostgreSQL beyond its in-built capabilities. If you’ve used SQL before, you’re likely familiar with built-in functions like COUNT(), AVG(), or SUM(). UDFs enable you to go beyond this and define your own custom behavior. This blog post will introduce you to the concept of UDFs in PostgreSQL, how to create and use them, and the various types and properties that UDFs have.

What are User-Defined Functions?

User-Defined Functions (UDFs) are named program units. They encapsulate a sequence of operations into a single callable unit. UDFs in PostgreSQL are a way to extend the functionality of the database server by writing functions in a language that PostgreSQL supports.

PostgreSQL provides support for various programming languages to write UDFs such as SQL, PL/pgSQL, C, Python, and Perl. These functions can perform operations like complex computations, executing queries, or even making network calls.

Here’s a simple example of a UDF in PostgreSQL:

CREATE FUNCTION add_two_numbers(a integer, b integer)
RETURNS integer AS $
BEGIN
   RETURN a + b;
END;
$ LANGUAGE plpgsql;

In the above code, add_two_numbers is a UDF that takes two integer parameters and returns their sum.

Creating User-Defined Functions

Creating a UDF in PostgreSQL involves using the CREATE FUNCTION statement. Here’s the general syntax:

CREATE FUNCTION function_name ([ [ argname ] [ argtype ] [, ...] ])
  RETURNS return_type
  LANGUAGE { SQL | language_name }
  [ VOLATILE | STABLE | IMMUTABLE ]
  AS $
    function_body
  $;

function_name: This is the name of the function.
argname: This is the name of the argument (optional).
argtype: This is the data type of the argument (required if an argument is given).
return_type: This is the data type of the value the function will return.
language_name: This is the programming language the function is written in. It could be SQL, plpgsql, python, etc.
VOLATILE | STABLE | IMMUTABLE: These are function volatility categories (we’ll discuss these in detail later).
function_body: This is the actual code of the function.

Let’s delve into the process of creating a simple function.

CREATE FUNCTION square(integer) RETURNS integer AS $
BEGIN
   RETURN $1 * $1;
END;
$ LANGUAGE plpgsql;

This function named square receives an integer as an argument and returns the square of it. $1 is used to reference the first argument passed into the function.

Function Volatility Categories

Function Volatility Categories in PostgreSQL are used to inform the system about the behavior of the function. It can be one of the following three categories:

VOLATILE: The function value can change even within a single table scan. This is the default category.
STABLE: The function’s value is guaranteed to remain the same for all rows in a single statement, but it might change across different SQL statements.
IMMUTABLE: The function’s value will never change, regardless of the inputs.

Here’s how you can define a function with the IMMUTABLE keyword:

CREATE FUNCTION add_two_numbers(a integer, b integer)
RETURNS integer AS $
BEGIN
   RETURN a + b;
END;
$ LANGUAGE plpgsql IMMUTABLE;

This tells PostgreSQL that the function add_two_numbers will always return the same output for the same inputs, and that it won’t do anything that could change the database state.

Calling User-Defined Functions

After creating a UDF, you can call it in various ways, including in a SELECT, INSERT, UPDATE, or DELETE statement.

Let’s see how to call the square function that we created:

SELECT square(7);

This will return 49, the square of 7.

Returning Tables

Sometimes, you may need your function to return a table, particularly when dealing with complex business logic that involves multiple tables. PostgreSQL allows you to define functions that return tables.

Here’s an example:

CREATE FUNCTION get_books_by_author(author_name text) RETURNS TABLE(book_id integer, book_title text) AS $
BEGIN
  RETURN QUERY SELECT id, title FROM books WHERE author = author_name;
END;
$ LANGUAGE plpgsql;

In this example, get_books_by_author is a function that takes an author’s name and returns a table containing the id and title of all books by that author.

To call this function, you would use the following SQL statement:

SELECT * FROM get_books_by_author('George Orwell');

This will return a table with two columns (book_id and book_title) containing information about all books by George Orwell.

PL/pgSQL Control Structures

In addition to SQL, you can use PL/pgSQL to write functions in PostgreSQL. PL/pgSQL is a block-structured language. The fundamental unit of PL/pgSQL is a block, which is defined by the BEGIN and END keywords.

PL/pgSQL provides several control structures, including conditional statements and loops, which are not available in standard SQL. Let’s look at a simple example:

CREATE OR REPLACE FUNCTION is_even(n integer) RETURNS text AS $
BEGIN
  IF n % 2 = 0 THEN
    RETURN 'Yes';
  ELSE
    RETURN 'No';
  END IF;
END;
$ LANGUAGE plpgsql;

The is_even function checks if a number is even and returns ‘Yes’ if it is, ‘No’ if it isn’t.

FAQ

1. What are user-defined functions in PostgreSQL?

User-Defined Functions (UDFs) are named program units in PostgreSQL. They encapsulate a sequence of operations into a single callable unit. UDFs in PostgreSQL are a way to extend the functionality of the database server by writing functions in a language that PostgreSQL supports.

2. How do I create a user-defined function in PostgreSQL?

You can create a UDF in PostgreSQL using the CREATE FUNCTION statement. This statement includes the function name, arguments, return type, the language the function is written in, function volatility category, and the actual function body.

3. Can a user-defined function in PostgreSQL return a table?

Yes, PostgreSQL allows you to define functions that return tables. This feature is particularly useful when dealing with complex business logic involving multiple tables.

4. What languages can I use to write user-defined functions in PostgreSQL?

PostgreSQL provides support for various programming languages to write UDFs, including SQL, PL/pgSQL, C, Python, and Perl.

5. What are function volatility categories in PostgreSQL?

Function Volatility Categories in PostgreSQL inform the system about the behavior of the function. They can be VOLATILE (the function value can change within a single table scan), STABLE (the function’s value is the same for all rows in a single statement), or IMMUTABLE (the function’s value will neverchange, regardless of the inputs).

6. How do I call a user-defined function in PostgreSQL?

After creating a UDF, you can call it in various ways, including in a SELECT, INSERT, UPDATE, or DELETE statement. For instance, if you have a function named square that takes an integer as an argument, you could call it as follows: SELECT square(7);

7. What is PL/pgSQL in PostgreSQL?

PL/pgSQL is a block-structured language for PostgreSQL that provides several control structures, including conditional statements and loops, which are not available in standard SQL. PL/pgSQL allows you to write more complex user-defined functions and is particularly useful for performing complex processing on the server side.

8. Can a user-defined function in PostgreSQL modify the database?

Yes, depending on how you write the function, a UDF in PostgreSQL can modify the database. However, it’s generally a best practice to have functions return values and use other SQL statements (like INSERT, UPDATE, DELETE) to modify the database.

Conclusion

User-defined functions in PostgreSQL provide a powerful way to encapsulate operations into callable units. They allow for complex server-side processing, extend the functionality of the database server, and can be written in various languages that PostgreSQL supports. With the ability to create UDFs that return scalars, sets, and even tables, you have a lot of flexibility in handling data within your PostgreSQL database.

The examples and explanations provided in this blog post will help you understand UDFs in PostgreSQL and start creating your own. Remember, the key to mastering UDFs, like any other programming concept, is practice. So, create your own UDFs, play around with them, and explore different possibilities.

User-Defined Functions in PostgreSQL