sql-server - T-SQL 中多个字段中的相同窗口函数 : optimization?

Question

我在 T-SQL SELECT 的多个 WHEN 中使用完全相同的窗口函数。

SELECT 
    ModuleUsage = CASE
        WHEN Operation = 1 and OperationResult=1 and 1 != LAG(operation) OVER(
            PARTITION BY moduleid, clientname
            ORDER BY moduleid, clientname, timestamp, operation
        )
            THEN 1
        WHEN Operation = 2 and OperationResult=3 and 1 = LAG(operation) OVER(
            PARTITION BY moduleid, clientname
            ORDER BY moduleid, clientname, timestamp, operation
        )
            THEN -1
        ELSE 0
    END
    , *
FROM [dbo].[LicencesTracing]

是否评估了两次，或者查询优化器是否第二次重用结果？ 提前致谢

Answer 1

我们可以像下面这样重写查询，以便只使用一次该函数吗？

       -- window function <> 1
SELECT CASE WHEN LAG(operation) OVER(PARTITION BY moduleid, clientname ORDER BY moduleid, clientname, timestamp, operation) <> 1

            THEN CASE WHEN Operation = 1 and OperationResult=1 THEN 1 ELSE 0 END

       -- window function = 1
       ELSE

            CASE WHEN Operation = 2 and OperationResult=3 THEN -1 ELSE 0 END
       END AS ModuleUsage
      ,*
FROM [dbo].[LicencesTracing];

我认为使用 IIF 会简化代码:

SELECT CASE WHEN LAG(operation) OVER(PARTITION BY moduleid, clientname ORDER BY moduleid, clientname, timestamp, operation) <> 1 
            THEN IIF(Operation = 1 and OperationResult=1, 1, 0)
            ELSE IIF(Operation = 2 and OperationResult=3, -1, 0)
       END AS ModuleUsage
      ,*
FROM [dbo].[LicencesTracing];