Execute the statement as follows: SELECT count (DISTINCT L1) from T1 where L1 in (select L2 from T2 where l3= ' xxx ') feels very slow (a few 10 seconds),
But performing subqueries individually select L2 from T2 where l3= ' xxx ' or removing subqueries this direct execution of select count (DISTINCT L1) from T1 is faster.
Changing the statement to join is much faster (millisecond level), with the following modified statements: SELECT COUNT (DISTINCT L1) from T1 inner join T2 where t1.l1=t2.l2 and t2.l3= ' xxx '.
Specific cause analysis visible: https://www.cnblogs.com/Bccd/p/6607515.html
The efficiency of MySQL nested query and connection query