I have this query:
SELECT DISTINCT t1.string FROM t1
LEFT JOIN t2 ON t1.string=t2.string
LEFT JOIN t3 ON t1.int_a=t3.int_a
LEFT JOIN t4 ON t1.int_b=t4.int_b
WHERE
t1.string != '' AND
t2.string IS NULL AND
t3.int_a = <some integer>
ORDER BY
t1.string ASC
This query is executing slower than it should. EXPLAIN has it using
temporary and using filesort.
I have indexes on every column in the query, but I think the problem
is the one-index-per-table limitation. According to EXPLAIN, there are
two possible indices in use for t1 (int_a and string), but only int_a
is being used. So I tried constructing a compound index on int_a and
string. Although this new index appears in possible_keys, EXPLAIN
still shows the key actually being used as int_a. I tried building the
compound key in both orders and had the same results. How do get mysql
to all possible keys on t1 when running the query? Thanks!
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