Question

The query below takes about 0.3 seconds. Why? And how can I solve it. The shouldnt take so long…

SELECT tshops.OfficialName
 , tresults.StartTime
 , tresults.Price
 , tresults.shopID
FROM
 tresults
INNER JOIN tshops
ON tshops.shopID = tresults.shopID
WHERE
  tresults.pID = 15
  AND tresults.websiteID = 1
  AND tresults.StartTime = (SELECT max(tresults.StartTime) AS maxstarttime
                        FROM
                          tresults
                        WHERE
                          tshops.shopID = tresults.shopID
                          AND tresults.pID = 15
                          AND tresults.websiteID = 1)
ORDER BY
  tresults.Price DESC

Explain result:

1, PRIMARY, tshops, ALL, PRIMARY, , , , 203, Using temporary; Using filesort
1, PRIMARY, tresults, eq_ref, PRIMARY,idxPID,idxWebsite,idxStartTimeASC,idxStartTimeDESC,fk_shopID, PRIMARY, 20, func,const,pricegrabber.tshops.shopID,const, 1, Using where
2, DEPENDENT SUBQUERY, tresults, ref, idxPID,idxWebsite,fk_shopID, fk_shopID, 4, pricegrabber.tshops.shopID, 891, Using where; Using index

So a full table scan on tshops is done. But why? There’s an index on shopID in both tables…

Answer 1

I would suspect the subselect which refers to the outer select is confusing matters.

Try doing a join against the subselect, using a GROUP BY on the shopID. Normally I would expect MySQL to optimise things to do this anyway, but worth a try.

Something like this (please excuse any typos)

SELECT tshops.OfficialName
 , tresults.StartTime
 , tresults.Price
 , tresults.shopID
FROM  tresults
INNER JOIN tshops
ON tshops.shopID = tresults.shopID
INNER JOIN (SELECT shopID, MAX(tresults.StartTime) AS MaxStartTime 
            FROM tresults 
            WHERE tresults.pID = 15 
            AND tresults.websiteID = 1 
            GROUP BY shopID) Sub1
ON tresults.StartTime = Sub1.MaxStartTime
WHERE
  tresults.pID = 15
  AND tresults.websiteID = 1
ORDER BY
  tresults.Price DESC