具有多个联接，max（）和分组依据的慢MySQL查询

Question

I've got a serious problem. Our Intranet is getting slower and slower. One of the mainreasons seems to be a slow mysql-query (it appears in the slow-query.log). That query is asked every time an intranet-site is opened. It looks like this:

SELECT w.Datetime, w.User_ID, w.Status, e.Lastname
FROM worktimes AS w
INNER JOIN employees AS e ON w.User_ID=e.ID
RIGHT JOIN (SELECT max(Datetime) AS Datetime, User_ID
            FROM worktimes
            WHERE Datetime>".$today." // variable of today 0.00 o'clock
            AND Location='".$llocation['ID']."' // variable of one of 9 locations
            GROUP BY User_ID) AS v
        ON v.User_ID=w.User_ID AND w.Datetime=v.Datetime
ORDER BY e.Lastname;

The worktimes-table is somewhat greater with up to 200k rows (momentary 90k to testing reasons) and 13 columns. The whole query goes through a loop with 3 to 9 cycles.

Has someone an idea how to make the queries faster?

edit: As wished here is the EXPLAIN-result.

id  select_type     table       type    possible_keys   key        key_len  ref               rows  Extra
1   PRIMARY         <derived2>  ALL     NULL            NULL       NULL     NULL              44006 Using temporary; Using filesort
1   PRIMARY         w           ALL     NULL            NULL       NULL     NULL              92378 Using where
1   PRIMARY         e           eq_ref  PRIMARY,ID      PRIMARY    4        ais_v1.w.User_ID      1 NULL
2   DERIVED         worktimes   ref     Location        Location   767      const             44006 Using index condition; Using where; Using temporary; Using filesort

Answer 1

you dont need to use worktimes twice

you can do it as follows:

SELECT max(w.Datetime) AS Datetime, w.User_ID, w.User_ID, w.Status, e.Lastname
        FROM worktimes w left outer join employees e
        on e.User_ID=w.User_ID 
        and w.Datetime>".$today." // variable of today 0.00 o'clock
     AND w.Location='".$llocation['ID']."' // variable of one of 9 locations
        GROUP BY w.User_ID
        ORDER BY e.Lastname;

it will run faster than your existing query

Answer 2

The w table needs an index on INDEX(Location, Datetime) . This should improve the performance.