PARTITION BY RANGE (used_at) (
    PARTITION p0 VALUES LESS THAN ('2012-01-01'),
    PARTITION p1 VALUES LESS THAN ('2013-01-01'),
    PARTITION p2 VALUES LESS THAN ('2014-01-01'),
);

WHERE used_at >= '2013-05-01' AND used_at < '2013-09-01'

user (3 000 000 records):
    user_id UNSIGNED INT ...
    ...


messages (50 000 000 records)
    sender UNSIGNED INT (refers to user)
    recipient UNSIGNED INT (refers to user)

WHERE ... (sender = 1234567 OR recipient = 1234567)
...
GROUP BY (sender + recipient)

PARTITION BY RANGE (sender + recipient) (
    PARTITION p0 VALUES LESS THAN (1000000),
    PARTITION p1 VALUES LESS THAN (2000000),
    ...
    PARTITION p5 VALUES LESS THAN (6000000),
);

WHERE ... (sender = 1234567 OR recipient = 1234567)