索引扫描，索引查找和表扫描

Question

I have following table:

Create table Tab_with_Ix
(
    id int not null,
    name nvarchar(10),
    phone decimal(10,0)
)

I inserted some records:

insert into Tab_with_Ix 
values(1, 'Yogesh', 8855664452), (2, 'Vinay', 9977884455), (3, 'Sam', 9988554466)

Now, I create a non-clustered index on Tab_with_Ix :

CREATE NONCLUSTERED INDEX NCI_Ix on Tab_with_Ix(id)

Now, when I query the Tab_with_Ix :

Select * 
from Tab_with_Ix
where id = 1

I get an execution plan as:

But, when I use the following query:

Select id 
from Tab_with_Ix 
where id = 1

I get an execution plan as:

My question is: why does SQL Server use a table scan once, and the other time, an index seek?

Answer 1

In case you do a SELECT * - you want all columns - so in the end, SQL Server must go back to the base table data. In such a case, often it's cheaper to just do a table scan (or clustered index scan) rather than an index seek with an expensive key lookup (or RID lookup, if no clustered index is present).

If you have a lot of rows, then at some point it will become more efficient for SQL Server to do an index scan and a single (or a few) key/RID lookups - so if you have thousands of rows in your sample table - at some point (the "tipping point"), SQL Server will start using your nonclustered index.

In the second case, when you do SELECT id , you only want the id column - and that column is in the index page - so an index seek on that index will give SQL Server all that it needs to satisfy this query - therefore, an index seek is typically much faster and will be preferred over the table scan.

This is one of the many reasons why you should try to avoid using SELECT * FROM dbo.Table as much as you can. With a SELECT * , more often than not, nonclustered indexes are not used, and a table (or clustered index) scan is used instead.