通过评估执行计划查询性能（加入第一行）

Question

我有以下表格：

客户

ID     Name 
============
1      John 
2      Alice
3      Bob

订单

ID     CustomerID   Status
==========================
1001       1          1
1002       2          1
1003       2          2
1004       3          2

我想加入表格，每个客户只显示一个条目（状态最低的那个），即

ID     Name    OrderID 
======================
1      John     1001
2      Alice    1002
3      Bob      1004

感谢this question 的回答，我选择了两个产生相同输出的解决方案：

解决方案 1

SELECT c.id, c.name, o.id FROM customers AS c
INNER JOIN orders AS o ON
c.id =  o.customerid
WHERE o.status = (SELECT MIN(status) FROM orders WHERE customerid = c.id)

解决方案 2

SELECT c.id, c.name, o.id FROM customers as c
INNER JOIN orders AS o ON
o.id = (SELECT TOP 1 id FROM orders WHERE customerid = c.id ORDER BY status)

为了了解哪个运行得更快，我使用了SQL Fiddle View Execution Plan，它给出了以下内容：

解决方案 1

解决方案 2

如何解释这些图表，哪一个执行得更快？ 使用 MS SQL Server 2016。

Answer 1

这是我的细分，最后一个是我给你的建议。

查询成本 67%

SELECT c.id, c.name, o.id FROM @Customers AS c
INNER JOIN @Orders AS o ON
c.id =  o.customerid
WHERE o.status = (SELECT MIN(status) FROM @Orders WHERE customerid = c.id)

查询成本 66%

SELECT c.id, c.name, o.id FROM @Customers as c
INNER JOIN @Orders AS o ON
o.id = (SELECT TOP 1 id FROM @Orders WHERE customerid = c.id ORDER BY status)

查询成本 47%

SELECT
  x.CustID,
  x.Name,
  x.OrderID
FROM (SELECT
  C.id CustID,
  c.Name,
  o.ID OrderID,
  o.status,
  ROW_NUMBER() OVER (PARTITION BY c.id ORDER BY o.status) rn
FROM @Customers c
INNER JOIN @Orders o
  ON o.CustomerID    = c.ID) x
WHERE x.rn = 1