Followers/following database structure

Question

My website has a followers/following system (like Twitter\'s). My dilemma is creating the database structure to handle who\'s following who.

What I came up with was crea

Answer 1

There is a better physical structure than proposed by other answers so far:

CREATE TABLE follower (
    user_id INT, -- References user.
    follower_id INT,  -- References user.
    PRIMARY KEY (user_id, follower_id),
    UNIQUE INDEX (follower_id, user_id)
);

InnoDB tables are clustered, so the secondary indexes behave differently than in heap-based tables and can have unexpected overheads if you are not cognizant of that. Having a surrogate primary key id just adds another index for no good reason¹ and makes indexes on {user_id, follower_id} and {follower_id, user_id} fatter than they need to be (because secondary indexes in a clustered table implicitly include a copy of the PK).

The table above has no surrogate key id and (assuming InnoDB) is physically represented by two B-Trees (one for the primary/clustering key and one for the secondary index), which is about as efficient as it gets for searching in both directions². If you only need one direction, you can abandon the secondary index and go down to just one B-Tree.

BTW what you did was a violation of the principle of atomicity, and therefore of 1NF.

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¹ And every additional index takes space, lowers the cache effectiveness and impacts the INSERT/UPDATE/DELETE performance.

² From followee to follower and vice versa.