MySQL长响应时间

Question

我有一个有效的MySQL查询，它从输入到我的DB中的每个社区中选择一个表的最新占用率，但是它似乎在扫描整个数据库条目，因为查找时间大约需要3-4秒。

在下面的查询中提供了详细信息，有人能为我提供一种更快/更好的方法来查找每个社区的最新时间戳字段吗？-我需要查询来选择输入的每个社区，并使用最新的时间戳，但是所选的每个社区的限制应该是1(意味着名为"Test community“的社区可能有数百份提交，但我需要选择最新输入的时间戳，以及表中输入的每个社区的相同选择)。

SELECT t1.reportID, t1.communityID, t1.region, t1.percentOccupied,  
t1.TIMESTAMP, Communities.fullName

FROM NightlyReports t1 

INNER JOIN Communities On t1.communityID = Communities.communityID

WHERE t1.TIMESTAMP = ( SELECT MAX( TIMESTAMP ) FROM NightlyReports WHERE 
t1.communityID = NightlyReports.communityID ) 

AND t1.region =  'GA' ORDER BY percentOccupied DESC

Answer 1

在我的经验中，相关子查询的性能通常很差；请尝试这样做：

SELECT t1.reportID, t1.communityID, t1.region, t1.percentOccupied
    , t1.TIMESTAMP, Communities.fullName
FROM NightlyReports AS t1 
INNER JOIN Communities ON t1.communityID = Communities.communityID
INNER JOIN (
   SELECT communityID, MAX( TIMESTAMP ) AS lastTimestamp
   FROM NightlyReports 
   WHERE region = 'GA'
   GROUP BY communityID
) AS lastReports ON t1.communityID = lastReports.communityID
                AND t1.TIMESTAMP = lastReports.lastTimestamp
WHERE t1.region =  'GA' 
ORDER BY percentOccupied DESC

Answer 2

你的查询很好。对于这个查询(只重写了一点)：

SELECT nr.reportID, nr.communityID, nr.region, nr.percentOccupied,  
       nr.TIMESTAMP, c.fullName
FROM NightlyReports nr INNER JOIN
     Communities c
     ON nr.communityID = c.communityID
WHERE nr.TIMESTAMP = (SELECT MAX(nr2.TIMESTAMP)
                      FROM NightlyReports nr2
                      WHERE nr.communityID = nr2.communityID
                     ) AND
     nr.region =  'GA'
ORDER BY percentOccupied DESC;

您需要索引：

• NightlyReports(region, timestamp, communityid)
• NightlyReports(communityid, timestamp)
• Communities(communityID) (可能已经存在)

关联子查询本身并不是一个问题。