sql-server - 使用 'where then Union' 或使用 'Union then Where'

Question

请记住以下两种类型的查询:

--query1
Select someFields
From someTables
Where someWhereClues
Union all
Select someFields
FROM some Tables
Where someWhereClues

--query2
Select * FROM (
    Select someFields 
    From someTables
    Union all 
    Select someFields
    FROM someTables
    ) DT
Where someMixedWhereClues

Note :
In both queries final result fields are same

我以为是第一个。查询速度更快或者性能更好!
但经过一些研究后，我对此感到困惑note :

SQL Server (as a sample of RDBMS) first reads whole data then seek records. => so in both queries all records will read and seek.

请帮助我解决我的误解，以及 query1 和 query2 之间是否还有其他差异？

<小时/>

编辑:添加示例计划:

select t.Name, t.type from sys.tables t where t.type = 'U'
union all
select t.Name, t.type from sys.objects t where t.type = 'U'

select * from (
    select t.Name, t.type from sys.tables t
    union all
    select t.Name, t.type from sys.objects t
    ) dt
where dt.type = 'U'

执行计划是:

both are same and 50%

Answer 1

SQL Server query optimizer ，优化两个查询，使您获得几乎相同的性能。