Question

This query takes a split second

select * from tbl1 limit 0,1

This query takes a second

SELECT Distinct(col2) FROM tbl2 WHERE col3 = 2

This query eats 100% cpu and takes 190 seconds duration (0s fetch) to return the result

select * from tbl1 WHERE ID IN (SELECT Distinct(col2) FROM tbl2 WHERE col3 = 2) limit 0,1

I am trying to run this query on the full data set (not just limiting it to one record)

What could be causing the performance problem, my table structures?

Answer 1

Subqueries in MySQL are notoriously slow. You can speed this up with a join:

SELECT A.*
FROM
    tbl1 A
    INNER JOIN (
        SELECT DISTINCT col2
        FROM
            tbl2
        WHERE
            col3 = 2
    ) X ON X.col2 = A.ID
LIMIT 0, 1

To update tbl1:

UPDATE
    tbl1 A
    INNER JOIN (
        SELECT DISTINCT col2
        FROM
            tbl2
        WHERE
            col3 = 2
    ) X ON X.col2 = A.ID
SET
    A.SomeCol = 'value'