# Postgresql – What type of database can I use to store mutual scores between every combination of users

nosqlpostgresql

Let's say I have an application that calculates and stores a mutual score between every combination of users.

I'm currently using Postgres to store this information.

So if my system had users with id 1, 2, and 3 that table might look like

CREATE TABLE scores (id,user_id,friend_id,score)
AS ( VALUES
( 1, 1, 2, 15.2 ),
( 2, 2, 1, 15.2 ),
( 3, 1, 3, 19.7 ),
( 4, 3, 1, 19.7 ),
( 5, 2, 3, 21.0 ),
( 6, 3, 2, 21.0 )
);


As you can see, the data itself is pretty simple.

• Every combination of users has a mutual score calculated between them (the derivation of that score or what it means isn't important here) and I store it in the table.
• Because I could look up either user, I have to store the pairings "both ways" (e.g. 1 -> 2 and 2 -> 1)

The problem is that this increases quadratically. With 10,000 users there are 100 million records in this table. With 100,000 users (which is a reasonable scope for my app) there would be 10 billion records.

So my question is – surely Postgres can't be the most scale-able way to store this information right? I don't need access to it constantly, just periodically when I'm running a batch job so I'm happy to store it in some separate type of DB on a separate server.

What kind of DB specializes in storing this type of paired data and can perform quick reads?

For example, I commonly query for all the ranked friends of a given user with SELECT friend_id FROM my_table where user_id = ? ORDER BY score DESC so I'd like to run the equivalent of that query on this new database.

Thanks!

P.S Someone had suggested a Graph DB – didn't know if that would apply here.

Because I could look up either user, I have to store the pairings "both ways"

Well, that's not true. You could normalize the data because the relations aren't directed anyway.

CREATE TABLE scores (
user_id   int,
friend_id int,
score     int,
CHECK ( user_id < friend_id ),
PRIMARY KEY (user_id, friend_id)
);

INSERT INTO scores (user_id, friend_id,score)
VALUES
( 1, 2, 15.2 ),
( 1, 3, 19.7 ),
( 2, 3, 21.0 );


Now you can find the relation easy.

SELECT *
FROM scores
WHERE user_id = 1
AND friend_id = 2;


If you don't want to put the rule in the app, just rewrite the query into.

SELECT *
FROM scores
WHERE (user_id, friend_id) =
ROW( least(1,2), greatest(1,2) );