Question

Query 1:

select distinct email from mybigtable where account_id=345

takes 0.1s

Query 2:

Select count(*) as total from mybigtable where account_id=123 and email IN (<include all from above result>)

takes 0.2s

Query 3:

Select count(*) as total from mybigtable where account_id=123 and email IN (select distinct email from mybigtable where account_id=345)

takes 22 minutes and 90% its in the “preparing” state. Why does this take so much time.

Table is innodb with 3.2mil rows on MySQL 5.0

Answer 1

After much deliberation and with help from dba.se, this is the final query that worked majic.

select count(*) EmailCount from
(
    select tbl123.email from
    (select email from mybigtable where account_id=123) tbl123
    LEFT JOIN
    (select distinct email from mybigtable where account_id=345) tbl345
    using (email)
    WHERE tbl345.email IS NULL
) A;