Question

When I first made this query and the table was relatively small, it ran very quickly, but over time as the update table size increased (now about 6000 rows) the query has been extrememly slow and resource intensive. The server I’m using is the 1GB RAM VPS from linode.com, and I haven’t actually waited long enough for the query to finish.

Interestingly, without the extra condition in the left join (SELECT MAX(u2.time)) it runs in < 0.5s.

I had a look in the process list of mySQL while the query was running, and it showed as ‘Sending Data’ for the whole time

Here’s the query:

SELECT 
  s.ID as sid, s.country AS country, 
  s.name AS name, s.ip AS ip, 
  u.connPlayers AS cp, u.maxPlayers AS mp
FROM servers AS s 

LEFT JOIN updates AS u 
  ON u.serverID = s.ID 
  AND u.time = 
      (SELECT MAX(u2.time) 
         FROM updates AS u2 
         WHERE u2.serverID = s.ID) 
ORDER BY RAND(MINUTE(NOW())) 
LIMIT 0,10

And here’s the my.cnf file: http://redream.co.nz/my.cnf

Table structure:

Servers table (20 rows
)

Field   Type
ID      int(10) Unique Key           
ip      varchar(200)                 
country varchar(2)               
name    varchar(600)             
motd    varchar(600)                 
desc    mediumtext               
version varchar(600)    

Update table (6000 rows)

Field       Type
serverID    int(10)              
ping        int(10)              
time        int(14)              
uptime      int(10)              
connPlayers int(10)              
maxPlayers  int(10)              
uptime      int(14)

Answer 1

I am no MySQL expert, but I think this will work for MySQL:

SELECT 
  s.ID as sid, s.country AS country, 
  s.name AS name, s.ip AS ip, 
  u.connPlayers AS cp, u.maxPlayers AS mp
FROM servers AS s 

LEFT JOIN (
    SELECT u.serverId, u.connPlayers, u.maxPlayers
    FROM updates u
    INNER JOIN (SELECT serverId, max(time) AS 'maxTime' FROM updates GROUP BY serverId) AS u2
        ON u.serverId = u2.serverId AND u.time = u2.maxTime 
) AS u ON u.serverID = s.ID 

ORDER BY RAND(MINUTE(NOW())) 
LIMIT 0,10

The important change is that the left join now contains the following subquery:

SELECT u.serverId, u.connPlayers, u.maxPlayers
FROM updates u
INNER JOIN (SELECT serverId, max(time) AS 'maxTime' FROM updates GROUP BY serverId) AS u2
    ON u.serverId = u2.serverId AND u.time = u2.maxTime 

This subquery again contains a subquery and returns for each serverId the row with the maximum time value. The above subquery is only executed once instead of for each row in the cross product.