Using JavaScript to write PostgreSQL functions

Posted on Sat Dec 12 2020

postgres javascript

If you’re new to PostgreSQL, just understand you can write functions or stored procedures to conveniently capture and reuse processes.

PostgreSQL includes it’s own lanaguge called PL/pgSQL which is an extension of the SQL language. It makes it more powerful by enabling things like variables and loops to write more versatile logic.

Here is a very primitive calculator function written in PL/pgSQL.

create or replace function calc_plpgsql(x int, y int, func text)
returns int
as
$$
declare
 result int = null;
begin
	if func = '+' then
		result =  x + y;
	elsif  func = '-' then
		result = x - y;
	elsif  func = '*' then
	result = x * y;
	elsif  func = '/' then
	result = x / y;
	else
		raise exception 'Invalid function provided';
	end if;
	
	return result;
end
$$
language plpgsql;

select calc_plpgsql(5,5,'+') -- returns 10

To break this down. First we’re creating a new function called calc_plpgsql. This takes three arguments x, y and func. x and y are both integers, and func will be a string which will signify the calculation function to perform e.g '+', '-' , '*' or /.

Note, we wrap our logic for the function in $$. This is known as dollar-quoted strings and is equivalent to template literals in JavaScript. It is a very common pattern when writing functions and stored procedures in PostgreSQL. It allows us to write any code between the $$’s without needing to escape single quotes and backslashes which can produce un-readable code.

Next, we can make use of if, elsif and else for conditional logic to detect what was passed into the func argument.

The last line is interesting. We state the language we want PostgreSQL to parse in the dollar quoted string. In this example we are using language plpgsql.

When I first saw this it got me thinking. “Does this mean we can use other languages too!?“. Turns out you can. PostgreSQL also supports other procedural languages such as Python, C and my favourite, JavaScript!

To be able to write JavaScript for your PostgreSQL function, you have to ensure that PL/v8 has been installed on your database server. If not, then there are a couple of additional steps required.

Once installed you can run:

CREATE EXTENSION plv8;

To test Pl/v8 ready, we can run:

SELECT plv8_version(); -- shoud show something similar to `3.0alpha`

Now lets rewrite our calculator function in JavaScript.

create or replace function calc_plv8(x int, y int, func text)
returns int
as
$$
if (func === '+'){
	return x + y
}
else if (func === '-'){
	return x - y
}
else if (func === '*'){
	return x * y
}
else if (func === '/'){
	return x - y
} else {
	plv8.elog(ERROR, 'invaid function');
}
$$
language plv8;

select calc_plv8(5,5,'+') -- returns 10

How fun is that?

Note, we’re using the V8 JavaScript engine inside the context on a database engine so this is not node.js or a browser. This means we don’t have access to all the APIs you may expect, such as console.log, instead you’ll need to use plv8.elog. Also don’t expect to be able to import libraries from NPM or anything! Nonetheless, I thought it was interesting to know this is possible.

If you want an easy way to test this out, I recommend spinning up a PostgreSQL database via https://supabase.io. It’s free!

Using JavaScript to write PostgreSQL functions

Further Reading