执行计划--WHERE条件的先后顺序对执行计划的影响

在编写SQL时，会建议将选择性高（过滤数据多）的条件放到WHERE条件的前面，这是为了让查询优化器优先考虑这些条件，减少生成最优(或相对最优)的执行计划的时间，但最终的执行计划生成过滤顺序还是决定这些条件的选择性与判断bool值的容易程度

测试代码：

GO
SELECT *
INTO #T1
FROM sys.all_columns
GO
SELECT *
INTO #T2
FROM sys.all_columns
GO
SELECT *
INTO #T3
FROM sys.all_columns
GO
SET STATISTICS IO ON
SET STATISTICS TIME ON
GO
SELECT * FROM #T1 AS T1
WHERE T1.[object_id]=3
AND (SELECT COUNT(1) FROM #T2 AS T2 WHERE T2.column_id>T1.column_id)>1

SELECT * FROM #T1 AS T1
WHERE (SELECT COUNT(1) FROM #T2 AS T2 WHERE T2.column_id>T1.column_id)>1
AND T1.[object_id]=3

执行计划：

可以从查询计划看出，无论T1.[object_id]=3在何处，其计算bool值相对简单，而(SELECT COUNT(1) FROM #T2 AS T2 WHERE T2.column_id>T1.column_id)>1 需要访问其他表，因此执行优化器优先执行T1.[object_id]=3，在满足T1.[object_id]=3为ture时再坚持行是否满足(SELECT COUNT(1) FROM #T2 AS T2 WHERE T2.column_id>T1.column_id)>1。

但对于以下查询：

SELECT * FROM #T1 AS T1
WHERE (SELECT COUNT(1) FROM #T3 AS T3 WHERE T3.[object_id]>T1.[object_id])<1
AND (SELECT COUNT(1) FROM #T2 AS T2 WHERE T2.column_id>T1.column_id)>1

SELECT * FROM #T1 AS T1
WHERE  (SELECT COUNT(1) FROM #T2 AS T2 WHERE T2.column_id>T1.column_id)>1
AND (SELECT COUNT(1) FROM #T3 AS T3 WHERE T3.[object_id]>T1.[object_id])<1

执行计划：

执行统计：

(25 row(s) affected)
Table 'Worktable'. Scan count 29, logical reads 36813, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#T3_________________________________________________________________________________________________________________00000000000C'. Scan count 4, logical reads 80, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#T1_________________________________________________________________________________________________________________000000000009'. Scan count 5, logical reads 59, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#T2_________________________________________________________________________________________________________________00000000000A'. Scan count 1, logical reads 59, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 111 ms,  elapsed time = 331 ms.

(25 row(s) affected)
Table '#T1_________________________________________________________________________________________________________________000000000009'. Scan count 5, logical reads 59, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 10731, logical reads 87653, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#T3_________________________________________________________________________________________________________________00000000000C'. Scan count 4, logical reads 236, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#T2_________________________________________________________________________________________________________________00000000000A'. Scan count 1, logical reads 59, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

(1 row(s) affected)

 SQL Server Execution Times:
   CPU time = 219 ms,  elapsed time = 281 ms.

条件 (SELECT COUNT(1) FROM #T2 AS T3 WHERE T3.[object_id]>T1.[object_id])<1 能过滤掉大部分数据，而(SELECT COUNT(1) FROM #T3 AS T2 WHERE T2.column_id>T1.column_id)>1不能过滤任何数据，因此如果优先执行(SELECT COUNT(1) FROM #T3 AS T2 WHERE T2.column_id>T1.column_id)>1，则会大大减少判断(SELECT COUNT(1) FROM #T2 AS T2 WHERE T2.column_id>T1.column_id)>1的次数，从而提高查询速度，但SQL Server无法推断出该结论，因此只能顺序判断WHERE 条件。

总结：虽然在很多情况下SQL Sever引擎能帮助我们判定 WHERE 条件后的执行顺序,但我们仍应该将选择性高(过滤数据多)的条件放置在 WHERE 语句中的前面，尤其对于复杂的SQL 语句，应仔细分析测试。

你比SQL SERVER 更了解你的数据！！！