MySQL LEFT JOIN with SUBQUERY is very slow

Question

In this query :

SELECT tblA.id FROM tblA 
LEFT JOIN (
        SELECT invid, max(id) as maxid, group_concat(testcase) as testgrp 
        FROM tblB 
        GROUP BY invid
        ) AS tblC ON tblC.invid = tblA.id 
LEFT JOIN  tblB on tblB.id = tblC.maxid 
WHERE 1 
GROUP BY tblA.id 

subquery is fast but with left join is very slow How can I make it faster?

Answer 1

By doing a subquery, the result of that subquery is loaded in to memory without any reference to indexes (like primary id or foreign keys, etc.). This way the ON statement is done without any index, which is slow.

You should create a query on which you join tblB on tblA, and do your group_concat on the result of both.

I'm not sure what your end result should be based on your example, so I can't provide you a query.

Answer 2

Looking to your code you could avoid a left join

SELECT tblC.id FROM (
        SELECT tblA.id, tblB.invid, max(tblB.id) as maxid, 
              group_concat(tblB.testcase) as testgrp 
        FROM  tblB 
        left  tblB ON  tblA.id  =  tblB.invid
        GROUP BY tblA.id, , tblB.invid
        ) AS tblC 
LEFT JOIN  tblB on tblB.id = tblC.maxid 

Answer 3

why not without any join:

SELECT
    a.id,
    (SELECT max(b.id) FROM tblB as b WHERE b.invid = a.id) as maxid,
    (SELECT group_concat(b.testcase) FROM tblB as b WHERE b.invid = a.id) as testgrp 
FROM tblA as a
WHERE 1