Question

To learn MySQL (innodb) I’m attempting to write a twitter app with 3 tables: users2, follow2, and tweets2.

Doing an explain as follows shows all the rows in tweets2 being possibly touched. Is this accurate? If you are going to do a WHERE username IN (select) I would assume that given username is indexed MySQL would retrieve the sub-select clause and go directly to the rows in tweets2 which have those usernames.

explain select username, timestamp, tweet
from tweets2
where username in 
  (select followee from follow2 where follower='user1') 
order by timestamp desc;

Heres the SQLAlchemy code creating the tables:

    users = Table('users2', metadata,
    Column('username', String(16), index=True),
    Column('email', String(256)),
    )
    follow = Table('follow2', metadata,
    Column('follower', String(16), index=True),
    Column('followee', String(16), index=True),
   )
    tweets = Table('tweets2', metadata,
    Column('id', Integer, primary_key=True),
    Column('username', String(16), index=True),
    Column('timestamp', BigInteger),
    Column('tweet', String(200)),
    )

Answer 1

In MySQL, the select statement inside the IN clause will always be driven (though the indexes may still be used on it).

Replace it with a JOIN:

SELECT  username, timestamp, tweet
FROM    follow2 f
JOIN    tweets2 t
ON      t.username = f.followee
WHERE   f.follower = 'user1'
ORDER BY
        timestamp DESC