我在查询具有父/子关系行的表时遇到问题。在编写一个简化示例时,我意识到 stackexchange 架构非常相似。
假设我正在通过 stackexchange 数据资源管理器查询 stackoverflow posts 表。我正在尝试获取所有帖子的子集及其相关答案。
帖子的子集是在一个 View 中定义的,它有一个相当复杂和昂贵的查询计划。在下面的示例中,它已被简化为只需选择前两行。
第一种方式,使用联合:
with ExpensiveView as (select top 2 ID from Posts order by ID)
select Posts.*
from ExpensiveView
left outer join Posts
ON ExpensiveView.Id = Posts.Id
union all
select Posts.*
from ExpensiveView
left outer join Posts
ON ExpensiveView.Id = Posts.ParentId
我很想避免这种方式,因为 ExpensiveView
被计算了两次。对于上面的简化版本显然不是问题,但会导致更复杂的问题。
第二种方式,使用带有条件连接子句的单个选择:
with ExpensiveView as (select top 2 ID from Posts order by ID)
select Posts.*
from ExpensiveView
left outer join Posts
ON ExpensiveView.Id = Posts.Id or ExpensiveView.Id = Posts.ParentId
这避免了 ExpensiveView
被评估两次,但会导致大得离谱的聚簇索引扫描。它似乎正在扫描 ExpensiveView
中每个 ID 的整个索引(因此 2 * 14977623 = ~3000 万行)。这非常慢。
两个问题
为什么第二个查询中的条件连接会导致如此大的索引扫描?
有什么方法可以在不对 ExpensiveView
进行多次计算的情况下获得我正在寻找的结果?
最佳答案
试试这个
with
ExpensiveView as (select top 2 ID from Posts order by ID),
CTE_Posts as (
select *, NP.Id as New_Post_ID
from Posts as P
outer apply (select P.Id union all select P.ParentId) as NP
)
select
P.*
from ExpensiveView as E
left outer join CTE_Posts as P on P.New_Post_ID = E.ID
关于sql-server - 使用条件连接语句时执行多个全索引扫描,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/17322676/