当 Distinct 和 Group By 的表现不同时?

问题描述

我知道在简单查询中，Distinct 和 Group By 的性能和执行计划几乎相同.

I know in simple queries the performance and execution plans of the Distinct and Group By are almost the same.

例如

SELECT Name FROM NamesTable GROUP BY Name
SELECT DISTINCT Name FROM NamesTable

但我在某些情况下读过他们的表现会有所不同，例如在子查询等中?

But I've read in some scenarios their performance would be different e.g. in subqueries, etc?

那么，您能否举一些例子或解释一些性能不同的场景?

So, could you make some examples or explain some scenarios where their performance are different?

非常感谢

推荐答案

如果在字段列表中包含计算值，您将看到执行计划中的差异.

If you include a calculated value in the field list you will see a difference in the execution plan.

select Value,
       getdate()
from YourTable
group by UnitID

select distinct
       Value,
       getdate()
from YourTable

group by 查询在计算标量值之前进行聚合.distinct 查询在聚合之前计算标量值.

The group by query aggregates before it computes the scalar value. The distinct query computes the scalar value before the aggregate.

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