MySQL 响应时间长

Question

我有一个有效的 MySQL 查询 select 来自每个社区的 table 的最新占用百分比进入我的数据库，但它似乎正在扫描整个数据库条目的查找时间大约需要 3-4 秒。

根据下面查询中提供的详细信息，有人可以为我提供一种 faster/better 方法来查找每个社区的最新时间戳字段吗？ - 我需要查询 select 每个进入的社区，具有最新的时间戳，但每个社区 selected 的限制应该是 1（这意味着名为 "Test Community" 的社区可能有数百个提交但我需要最新输入的时间戳 selected，以及 table)

中输入的每个社区的相同 selection

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

根据我的经验，相关子查询的性能通常很差；试试这个：

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

您的查询没问题。对于此查询（仅重写了一点）：

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)（这可能已经存在）

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