Question

尽管以下查询适合我的使用，但我试图了解是否有一种优化方法，因为它在UNION的两个语句中使用相同的嵌套子查询。我的直觉说，应该有一种方法可以查询一次子查询，并将结果用于UNION的两个部分，但是当尝试在UNION中使用联接时，我遇到了语法问题。

SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
    FROM dbo.ClientLocal
    WHERE clientLocalID = 
        ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%0123456789')
UNION
SELECT ClientMasterName, '0', clientMasterID, 2
    FROM dbo.ClientMaster
    WHERE clientMasterID = 
        ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%0123456789')
ORDER BY HierarchyType ASC;

为帮助解释为什么需要原始查询的原因，请设想“本地”客户端和“主”客户端的概念-本地客户端与主客户端绑定，但是主客户端可能有也可能没有本地客户。同样，与每个客户端关联的电话号码仅应绑定到本地客户端或主客户端（因此，在第二条语句中，本地客户端ID的硬编码为“ 0”-因为没有关联）。

这将针对Microsoft SQL Server 2008 R2运行。

Answer 1

如果您不喜欢重复代码，则可以使用公用表表达式：

WITH cte AS (
  SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%0123456789'
)
SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
FROM dbo.ClientLocal
WHERE clientLocalID IN (SELECT HierarchyItem FROM cte)
UNION
SELECT ClientMasterName, '0', clientMasterID, 2
FROM dbo.ClientMaster
WHERE clientMasterID IN (SELECT HierarchyItem FROM cte)
ORDER BY HierarchyType ASC;

Answer 2

由于似乎没有人提及它：减少代码并不一定意味着它也将更快地执行。人们似乎常常认为，首先执行CTE（如果有多个，则“按顺序”执行），然后应用最终查询。他们不是。实际上，服务器将在很大程度上处理cte，就好像它是FROM或JOIN子句中某个位置的派生表一样，并选择完全相同的查询计划来执行它们。为了避免系统不得不扫描ClientPhone表两次（LIKE '%0123...'是非常“烦人的”操作），您必须先做一次，将结果存储在临时表中，然后然后在您的实际查询中使用所述临时表。

为此，我随意创建了一些示例数据（请参见下面的代码）。我的测试结果是：

原始查询耗时71ms
CTE版本耗时71ms，并使用完全相同的查询计划
创建临时表需要26ms，使用它的查询需要23ms

因此，即使服务器需要创建临时表并填充它，合并后的临时表仍只需49毫秒，而不是原来的71毫秒！

当然，根据所涉及的记录数和重复查询的复杂程度，您的里程可能会有所不同，就像我说的那样，LIKE '%blah'是一件令人讨厌的事情，因为它需要对表进行全面扫描（或（索引）有点运气。如果是WHERE pk_field = @value，效果可能会大不相同）

查询愉快...

IF DB_ID('test') IS NULL CREATE DATABASE test
GO
USE test
GO

-- setup
IF OBJECT_ID('ClientPhone') IS NOT NULL DROP TABLE ClientPhone 
IF OBJECT_ID('ClientLocal') IS NOT NULL DROP TABLE ClientLocal
IF OBJECT_ID('ClientMaster') IS NOT NULL DROP TABLE ClientMaster
GO
SELECT TOP 50000 
       HierarchyItem = IDENTITY(int, 1, 1),
       PhoneNumber   = Convert(varchar(100), NewID())
  INTO dbo.ClientPhone 
  FROM sys.objects x1, sys.columns x2, sys.objects x3, sys.columns x4

SELECT ClientLocalName = 'client dummy', 
       ClientLocalID   = Convert(int, Rand(HierarchyItem * 37) * 50000), 
       ClientMasterID   = Convert(int, Rand(HierarchyItem * 47) * 50000), 
       HierarchyType   = 1
  INTO dbo.ClientLocal
  FROM dbo.ClientPhone 

SELECT ClientMasterName = 'master dummy', 
       ClientLocalID    = Convert(int, Rand(HierarchyItem * 51) * 50000), 
       ClientMasterID   = Convert(int, Rand(HierarchyItem * 53) * 50000), 
       HierarchyType    = 2
  INTO dbo.ClientMaster
  FROM dbo.ClientPhone 

GO
-- original 
SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
  FROM dbo.ClientLocal
 WHERE ClientLocalID = ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%01')
UNION
SELECT ClientMasterName, '0', ClientMasterID, 2
  FROM dbo.ClientMaster
 WHERE ClientMasterID = ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%01')
ORDER BY HierarchyType ASC;

-- CTE
;WITH cte
   AS (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%01')

SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
  FROM dbo.ClientLocal
 WHERE ClientLocalID = ANY (SELECT HierarchyItem FROM cte)
UNION
SELECT ClientMasterName, '0', ClientMasterID, 2
  FROM dbo.ClientMaster
 WHERE ClientMasterID = ANY (SELECT HierarchyItem FROM cte)
ORDER BY HierarchyType ASC;

-- temp table
IF OBJECT_ID('tempdb..#ClientPhone') IS NOT NULL DROP TABLE #ClientPhone

SELECT HierarchyItem INTO #ClientPhone FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%01'

SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
  FROM dbo.ClientLocal
 WHERE ClientLocalID = ANY (SELECT HierarchyItem FROM #ClientPhone)
UNION
SELECT ClientMasterName, '0', ClientMasterID, 2
  FROM dbo.ClientMaster
 WHERE ClientMasterID = ANY (SELECT HierarchyItem FROM #ClientPhone)
ORDER BY HierarchyType ASC;

Answer 3

;with subQry as (
    SELECT ClientLocalName, ClientLocalID, ClientMasterID
        FROM dbo.ClientLocal
        WHERE clientLocalID = 
            ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%0123456789')
)
SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
    FROM subQry
UNION
SELECT ClientMasterName, '0', clientMasterID, 2
    FROM subQry
ORDER BY HierarchyType ASC;

Answer 4

我可以看到的唯一明显的优化是使用union all而不是union：

SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
FROM dbo.ClientLocal
WHERE clientLocalID = ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%0123456789')
UNION ALL
SELECT ClientMasterName, '0', clientMasterID, 2
FROM dbo.ClientMaster
WHERE clientMasterID = ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%0123456789')
ORDER BY HierarchyType ASC;

显然，子选择之间没有重复项（由于最后一列）。假设每个子选择中都没有重复项。

我不知道SQL Server对= ANY的处理方式，但是我认为它与IN基本上相同。 = ANY子查询将运行一次（对于每个子查询），但是第二次将结果缓存。您可以将其简化为：

SELECT ClientLocalName, ClientLocalID, ClientMasterID, 1 HierarchyType
FROM ((SELECT ClientLocalName, ClientLocalID, ClientMasterID
       FROM dbo.ClientLocal
      ) UNION ALL
      (SELECT ClientLocalName, '0', ClientMasterID
       FROM dbo.ClientMasterName
      ) 
     ) c        
WHERE c.clientLocalID = ANY (SELECT HierarchyItem FROM dbo.ClientPhone WHERE PhoneNumber LIKE '%0123456789')
ORDER BY HierarchyType ASC;

我不知道这是否会有很大的改善。

LIKE模式在数字的开头有一个通配符，使其难以优化。

SQL：使用冗余联合代码优化嵌套查询？

4 个答案: