Re: Slow COUNT + GROUP BY
Unless your "day" column is your primary key, that FORCE INDEX is going to force a full table scan every time. Get rid of it, and index the "day" column.
Index (day,uid) if uid isn't the PK in that table.
Also, consider rephrasing the inner query SELECT COUNT(DISTINCT uid) FROM ... so you can get rid of the outer query and do it all in one logical pass and make the optimizer's job a little easier.
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Re: Slow COUNT + GROUP BY
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