为什么 SQL Server 中非聚集索引和聚集索引的执行计划存在差异？

Question

Please explain why the below differences between non clustered and clustered index. First I am running the below two select statements.

select * 
from [dbo].[index_test2]  
where id = 1 -- Nonclustered index  on id column

select * 
from [dbo].[index_test1] 
where id = 1  -- Clustered index on id column

Execution plan shows "Table scan" for the first query and "Clustered index seek (clustered)" for the second query.

Then I am running below two statements.

select id 
from [dbo].[index_test2]  
where id = 1 -- Nonclustered index  on id column

select id 
from [dbo].[index_test1] 
where id = 1  -- Clustered index on id column

Execution plan shows "Index seek (NonClustered)" for the first query and "Clustered index seek (Clustered)" for the second query.

You can see from the above two cases, when using clustered index it is going for "Index seek" but for in case of NonClustered index it shows "Table scan" (executed with *) and it shows "Index seek (NonClustered)" (executing with index applied column-id).

Can any one clarify why the NonClustered index reacting differently on both cases?

Answer 1

A clustered index defines the order in which data is physically stored in a table but A non-clustered index doesn't sort the physical data inside the table.In fact, a non-clustered index is stored at one place and table data is stored in another place.

If you use an Non-Clustered Index it works in Index seek (NonClustered) mode when you call it property,but If you put where in Non-Clustered Index mode but call in select more expressions that are not Cover index change mode to Table scan

Indexes with included columns provide the greatest benefit when covering the query. This means that the index includes all columns referenced by your query, as you can add columns with data types, number or size not allowed as index key columns

But in Clustered Index , since the actual sorting is done by it, you do both in Clustered index seek (clustered) mode.