Question

我有一个有效的MySQL查询，它从我的数据库中输入的每个社区中选择表的最新占用百分比，但它似乎扫描整个数据库，因为查找时间大约需要3-4秒。

通过下面的查询中提供的详细信息，有人可以为我提供更快/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，但所选社区的限制应为1（意味着名为“测试社区”的社区可能有数百个提交，但我需要选择最新输入的时间戳，对于在表格中输入的每个社区，选择相同的选项）

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，相关子查询通常具有相当差的性能;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（稍微重写）：

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;

您想要索引：

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

MySQL响应时间长

2 个答案: