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