我有一个查询,使用 FULL JOIN 需要 2.5 秒,使用 INNER、RIGHT 或 LEFT JOIN 需要 40 秒。
这是查询。子查询(完成两次)本身只需要 1.3 秒。
SELECT T1.[time], T1.Total, T1.rn, T2.[time], T2.Total, T2.rn
FROM
(
select [time], MAX(ComputedValue) as Total, row_number() over (order by [time]) as rn
FROM
(
select SUBSTRING(CONVERT(CHAR(10), IntervalStartTime, 108), 0, 6) as [time], ComputedValue
from LoadTestTransactionSample
where LoadTestRunId=285
and CounterName='Total Transactions'
and TransactionName='Export'
) foo
group by [time]
) T1
_____ JOIN
(
select [time], MAX(ComputedValue) as Total, row_number() over (order by [time]) as rn
FROM
(
select SUBSTRING(CONVERT(CHAR(10), IntervalStartTime, 108), 0, 6) as [time], ComputedValue
from LoadTestTransactionSample
where LoadTestRunId=285
and CounterName='Total Transactions'
and TransactionName='Export'
) foo
group by [time]
) T2
ON T1.rn = T2.rn - 1
The select SUBSTRING
bit 只是从 DateTime 中获取 HH:MM 字符串。LoadTestTransactionSample
实际上是一个连接 8 个表的 VIEW。 (仅供参考,该数据库是 Visual Studio 负载测试结果存储)。以下是其(相关)列:
LoadTestRunId INT NOT NULL
CounterName NVARCHAR(255) NOT NULL
TransactionName NVARCHAR(64) NOT NULL
IntervalStartTime DATETIME NOT NULL
IntervalEndTime DATETIME NOT NULL
ComputedValue REAL
FULL JOIN 返回额外不需要的行,因此我确实需要执行 RIGHT JOIN 才能获得正确的答案。
我并不是真的在寻找解决方案(我有一个:将子查询预取到表变量使用SQL Server 2012分析函数'LAG',谢谢@a1ex07),只是一些了解什么可能导致这些连接类型之间的性能存在巨大差异.
EDIT:这是慢速右连接查询计划 http://sdrv.ms/10hzuky和快速全连接查询计划 http://sdrv.ms/10hB2Lk。它们太大了,无法发布屏幕截图。
EDIT 2:实际上,查询计划的 RIGHT JOIN 为 45%,FULL JOIN 为 55%,结果证明这是完全不准确的(实际上它最终比 99%/1% 更糟糕)。我想这意味着我必须依赖实际的执行统计数据。
EDIT 3:慢速 RIGHT JOIN 的统计:
(40 row(s) affected)
Table 'LoadTestPerformanceCounterCategory'. Scan count 0, logical reads 37556, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestPerformanceCounter'. Scan count 0, logical reads 176464, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestScenario'. Scan count 0, logical reads 176464, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestCase'. Scan count 0, logical reads 176464, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'WebLoadTestTransaction'. Scan count 0, logical reads 13411100, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestPerformanceCounterInstance'. Scan count 0, logical reads 36563718, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestPerformanceCounterSample'. Scan count 19721, logical reads 269657, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestRunInterval'. Scan count 41, logical reads 205, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 36754 ms, elapsed time = 36763 ms.
快速 FULL JOIN 的统计:
(41 row(s) affected)
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestPerformanceCounterCategory'. Scan count 0, logical reads 1832, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestPerformanceCounter'. Scan count 0, logical reads 8608, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestScenario'. Scan count 0, logical reads 8608, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestCase'. Scan count 0, logical reads 8608, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'WebLoadTestTransaction'. Scan count 0, logical reads 654200, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestPerformanceCounterInstance'. Scan count 0, logical reads 1783596, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestPerformanceCounterSample'. Scan count 962, logical reads 13154, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'LoadTestRunInterval'. Scan count 2, logical reads 10, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 1950 ms, elapsed time = 1944 ms.
尽管查询计划明显相似,但 RIGHT JOIN 比 FULL JOIN 执行的读取和扫描要多得多。
Is 工作表(在 FULL JOIN 中)提示?那是临时表吗?
这似乎表明查询优化器已损坏?