SELECT行的最有效方法WHERE ID EXISTS IN第二个表

Question

我希望从第二个表格中存在ID的一个表中选择所有记录。

以下两个查询返回正确的结果：

查询1：

SELECT *
FROM Table1 t1
WHERE EXISTS (SELECT 1 FROM Table2 t2 WHERE t1.ID = t2.ID)

查询2：

SELECT *
FROM Table1 t1
WHERE t1.ID IN (SELECT t2.ID FROM Table2 t2)

其中一个查询是否比另一个更有效？我应该使用一个吗？有没有第三种方法我没想到会更有效率？

Answer 1

摘要：

  

IN和EXISTS在所有情况下的表现相似。以下是用于验证的参数..

     

执行成本，时间：
  两者相同，优化器生成相同的计划。
   内存授予：
  两个查询都相同    Cpu时间，逻辑读取：
  尽管读取相同，但存在似乎在CPU时间方面略微超过IN。

我使用下面的测试数据集运行每个查询10次..

1. 非常大的子查询结果集（100000行）
2. 重复行
3. 空行

对于上述所有情况，IN和EXISTS都以相同的方式执行。

有关用于测试的Performance V3 database的一些信息。 20000个客户拥有1000000个订单，因此每个客户在订单表中随机复制（范围为10到100个）。

执行成本，时间：
下面是两个运行查询的屏幕截图。观察每个查询的相对成本。



内存成本：
两个查询的内存授权也是相同的..我强制MDOP 1，以免将它们泄露给TEMPDB ..



CPU时间，读取：

对于存在：

Table 'Workfile'. 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 '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 'Customers'. Scan count 1, logical reads 109, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Orders'. Scan count 1, logical reads 3855, 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 = 469 ms,  elapsed time = 595 ms.
SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

对于IN：

(20000 row(s) affected)
Table 'Workfile'. 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 '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 'Customers'. Scan count 1, logical reads 109, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Orders'. Scan count 1, logical reads 3855, 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 = 547 ms,  elapsed time = 669 ms.
SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 0 ms.

在每种情况下，优化器都足够智能，可以重新排列查询。

我倾向于使用EXISTS（我的意见）。使用EXISTS的一个用例是当您不想返回第二个表结果集时。

根据Martin Smith的查询进行更新：

我运行了以下查询，以找到从第一个表中获取第二个表中存在引用的行的最有效方法。

SELECT DISTINCT c.*
FROM Customers c
JOIN Orders o ON o.custid = c.custid   

SELECT c.*
FROM Customers c
INNER JOIN (SELECT DISTINCT custid FROM Orders) AS o ON o.custid = c.custid

SELECT *
FROM Customers C
WHERE EXISTS(SELECT 1 FROM Orders o WHERE o.custid = c.custid)

SELECT *
FROM Customers c
WHERE custid IN (SELECT custid FROM Orders)

除了第二个INNER JOIN之外，上述所有查询共享相同的费用，其余部分的计划相同。



内存授予：
此查询

SELECT DISTINCT c.*
FROM Customers c
JOIN Orders o ON o.custid = c.custid 

所需的内存授予量



此查询

SELECT c.*
FROM Customers c
INNER JOIN (SELECT DISTINCT custid FROM Orders) AS o ON o.custid = c.custid 

所需的内存授予..



CPU时间，读取：
对于查询：

SELECT DISTINCT c.*
FROM Customers c
JOIN Orders o ON o.custid = c.custid   

(20000 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 'Workfile'. Scan count 48, logical reads 1344, physical reads 96, read-ahead reads 1248, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Orders'. Scan count 5, logical reads 3929, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Customers'. Scan count 5, logical reads 322, 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 = 1453 ms,  elapsed time = 781 ms.

查询：

SELECT c.*
FROM Customers c
INNER JOIN (SELECT DISTINCT custid FROM Orders) AS o ON o.custid = c.custid

(20000 row(s) affected)
Table 'Customers'. Scan count 5, logical reads 322, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
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 'Workfile'. 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 'Orders'. Scan count 5, logical reads 3929, 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 = 1499 ms,  elapsed time = 403 ms.

Answer 2

它们在纯SQL语法中的含义完全相同，因此更“高效”取决于编译器如何构建计划，以及引擎如何获取数据。唯一可以确定的方法是双向运行并比较结果。

即便如此，差异仅适用于在该上下文中，这意味着可能存在另一种情况，其他情况更快。

Answer 3

一般情况相同但是当一个人做得更好时，存在赢了（对我而言）

如果t1.id是唯一的，那么你可以只做一个连接

SELECT t1.*
FROM Table1 t1
JOIN Table2 t2 
ON t1.ID = t2.ID 

即使t2.ID不是唯一的，你只需要唯一的行，那么你可以

SELECT distinct t1.*
FROM Table1 t1
JOIN Table2 t2 
ON t1.ID = t2.ID