Which non-clustered index should I use?

Question

Here I am studying nonclustered indexes on SQL Server Management Studio.

I've created a table with more than 1 million records. This table has a primary key.

CREATE TABLE [dbo].[Customers](
    [CustomerId] [int] IDENTITY(1,1) NOT NULL,
    [CustomerName] [varchar](100) NOT NULL,
    [Deleted] [bit] NOT NULL,
    [Active] [bit] NOT NULL,
 CONSTRAINT [PK_Customers] PRIMARY KEY CLUSTERED 
(
    [CustomerId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF,     ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

This is the query I'll be using to see what execution plan is showing:

SELECT CustomerName FROM Customers

Well, executing this command with no additional non-clustered index, it leads the execution plan to show me:

I/O cost = 3.45646
Operator cost = 4.57715

Now I'm trying to see if it's possible to improve performance, so I've created a non-clustered index for this table:

1) First non-clustered index

CREATE NONCLUSTERED INDEX [IX_CustomerID_CustomerName] ON [dbo].[Customers] 
(
    [CustomerId] ASC,
    [CustomerName] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF,
IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON,
ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
GO

Executing again the select against Customers table, the execution plan shows me:

I/O cost = 2.79942
Operator cost = 3.92001

It seems better. Now I've deleted this just created non-clustered index, in order to create a new one:

2) First non-clustered index

CREATE NONCLUSTERED INDEX [IX_CustomerIDIncludeCustomerName] ON [dbo].[Customers] 
(
    [CustomerId] ASC
)
INCLUDE ( [CustomerName]) WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF,  
SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF,   
ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
GO

With this new non-clustered index, I've executed the select statement again and the execution plan shows me the same result:

I/O cost = 2.79942
Operator cost = 3.92001

So, which non-clustered index should I use? Why the costs are the same on execution plan for I/O and Operator? Am I doing something wrong or this is expected?

thank you

Answer 1

This is because of "CustomerName" being INCLUDE-ed in the second index (see this about INLCUDEd columns).

Basically, BOTH indexes work the same exact way for you - they are covered indexes with the first index column NOT matching the WHERE clause.

Meaning that the query in both cases will be scanning the index but NOT touching the table.

The index that I'd expect ti be better performing for that specific query would be an index on CustomerName alone.