Query Optimizer Problems

[laurenty]

Hi,

I have a simple query that looks like
SELECT t.siteid FROM table t WHERE t.userid IN (1,2,3)

when running an EXPLAIN on this query I get
possible_keys: userid, userid_3
key: studentid_2
ref: NULL

the query should be using userid_3 (userid, siteid) instead of studentid_2 (studentid, userid, siteid, year)
The studentid_2 index is twice a big as the userid_3 index.

I've ran analyze, optimize, check, and repair on the table and it made no difference.
any ideas???

Thanks,

Laurent