How does Facebook implement friend connections at the database level? Do they have a table of friends per user?

Answer: Each user is sharded on to a single User Database (UDB). Each UDB has a "friends" table with columns id1, id2 where id1 is the primary key. id1 is always a user that is on that UDB, while id2 is the foreign user. To get all of a user's friends, you SELECT * from friends where id1= fro...

[tgk]

ユーザマスタをshardingしたら、ユーザ同士の再帰表もshardingしないとスケールしない、とすればaとbがfriendだという情報はa側b側で二重持ちになる