[英]Why are these two SQL queries so different in efficiency?
我必须在实习中使用 SQL,虽然我知道它的要点,但我并没有真正的编程背景,也不知道是什么使代码高效等。
查询#1
SELECT DISTINCT
c.[STAT], c.[EVENT], f.[STAT], f.[EVENT]
FROM
(SELECT *
FROM
(SELECT
*,
ROW_NUMBER() OVER (PARTITION BY [ID] ORDER BY [PROCDT], [PROCTIME]) AS a
FROM
TABLE) AS b
) AS c
LEFT JOIN
(SELECT
*
FROM
(SELECT
*,
ROW_NUMBER() OVER (PARTITION BY [ID] ORDER BY [PROCDT], [PROCTIME]) AS d
FROM
TABLE) AS e
) AS f ON c.[ID] = f.[ID] AND a = d - 1
ORDER BY
c.[STAT], c.[EVENT], f.[STAT], f.[EVENT]
查询#2
SELECT DISTINCT
b.[STAT], b.[EVENT], d.[STAT], d.[EVENT]
FROM
(SELECT
*,
ROW_NUMBER() OVER (PARTITION BY [ID] ORDER BY [PROCDT], [PROCTIME]) AS a
FROM TABLE) AS b
LEFT JOIN
(SELECT
*,
ROW_NUMBER() OVER (PARTITION BY [ID] ORDER BY [PROCDT], [PROCTIME]) AS c
FROM TABLE) AS d ON b.[ID] = f.[ID] AND a = c - 1
ORDER BY
b.[STAT], b.[EVENT], d.[STAT], d.[EVENT]
查询 #1 和 #2 返回相同的结果,这是预期的,但查询 #1 的运行时间大约为 5 秒,而查询 #2 的运行时间大约为 1 分 35 秒。 换句话说,第二个查询的运行时间比第一个查询多 1.5 分钟,我真的很想知道为什么。
编写此查询的正确方法是使用lead()
。 我很确定不需要select distinct
,所以这可以满足您的需求:
SELECT stat, event,
LEAD(stat) OVER (PARTITION BY ID, ORDER BY PROCDT, PROCTIME) as next_stat,
LEAD(event) OVER (PARTITION BY ID, ORDER BY PROCDT, PROCTIME) as next_event
FROM TABLE t
ORDER BY stat, event;
您编写的两个查询在 SQL Server 中应该相同。 显然,额外的子查询混淆了优化器。 您需要了解执行计划才能更好地理解这一点。
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