Very Slow MYSQL Sub Query

问题

Guys, im more of a MSSQL guy but im working on some MYSQL right now.

Iv written a simple query, with a subquery and I cant understand for the life of me why its so slow.

This query:

   SELECT MAX(timestamp), user, status FROM checkin WHERE room_id = 'Room Name' AND timestamp        > DATE_SUB(Now() ,INTERVAL 4005 SECOND) GROUP BY user

Runs in 0.0034 seconds

Yet this relatively similiar query but nested, takes over 6 seconds ..

SELECT user, status FROM checkin
WHERE timestamp IN
(SELECT MAX(timestamp) FROM checkin WHERE room_id = 'Room Name' AND timestamp > DATE_SUB(Now() ,INTERVAL 4005 SECOND) GROUP BY user)

Can anyone please help? Im stuck.

The table "checkin" only has about 900 rows in it. only the room_id column is indexed.

Cheers

EDIT Thanks guys .. heres the result of the EXPLAIN

DEPENDENT SUBQUERY checkin ref room_id room_id 202 const 1104 Using where; Using temporary; Using filesort

回答1:

Look into using a HAVING clause to achieve the same results. MySQL is notoriously bad at sub-query optimization, try this:

SELECT MAX(timestamp) as ts, user, status 
FROM checkin
WHERE room_id = 'Room Name' 
AND   timestamp > DATE_SUB(Now() ,INTERVAL 4005 SECOND)
GROUP BY user
HAVING timestamp = ts

also make sure that there is an index on timestamp

Alternatively:

SELECT user, status 
FROM checkin
WHERE room_id = 'Room Name' 
AND   timestamp > DATE_SUB(Now() ,INTERVAL 4005 SECOND)
AND NOT EXISTS (SELECT * FROM checkin as newer 
                WHERE newer.timestamp>checkin.timestamp
                AND newer.room_id = 'Room Name'
                AND newer.user = checkin.user)
GROUP BY user

回答2:

I think, that you are dealing with dependent subquery. This means, that subquery is not executed once (as you and I would expect), but for every row - it is a known bug of MySQL. If you can, split it into two queries - find out the MAX value first and then do a selection.

回答3:

Please run EXPLAIN on both queries. Propably you dont have proper indexes on your columns.

Try that:

EXPLAIN SELECT user, status FROM checkin WHERE timestamp IN (SELECT MAX(timestamp) FROM checkin WHERE room_id = 'Room Name' AND timestamp > DATE_SUB(Now() ,INTERVAL 4005 SECOND) GROUP BY user);

And:

SELECT MAX(timestamp) FROM checkin WHERE room_id = 'Room Name' AND timestamp > DATE_SUB(Now() ,INTERVAL 4005 SECOND) GROUP BY user

来源：https://stackoverflow.com/questions/5960901/very-slow-mysql-sub-query

标签

mysql

subquery