Impact of Nonclustered Index without Clustered Index - SQL Server by serverku

We have read and learn so many times from online sources as it is not best practice to create a non-clustered index without any clustered index created on the table. We should have clustered index on the table.

Have you practically seen that best practice? What will be the impact on query when table have a non-clustered index but not clustered index?. Without a clustered index on the table the query execution plan sometime used non-clustered index and sometimes not as depends on where condition used in the query.

I will show you here, how it behaves with and without clustered index. We have a two shot of the show and will see this behavior with normal non-clustered index and additional covering non-clustered index. Let we created a sample table and get insert some records in it.

 
--- Creating tables
IF (OBJECT_ID('ItemDetails','U') > 0 )
 DROP TABLE ItemDetails
GO

CREATE TABLE ItemDetails
(
 ItemAutiId int identity(1,1),
 ItemId int not null,
 ItemName varchar(100),
 ItemType varchar(10),
 ItemDescription varchar(200)
)
GO

-- Inserting sample records
INSERT INTO ItemDetails
SELECT 
 a.id,
 a.name,
 a.xtype,
 'ItemDesc' 
FROM sys.sysobjects a
CROSS JOIN sys.sysobjects b
GO

As discussed, we will go through the following, Will review execution plan for each and compare.

1. Normal non-clustered index behavior without and with clustered index on table.

-- Creating nonclustered index
CREATE NONCLUSTERED INDEX [IX_ItemType]
ON [dbo].[ItemDetails] ([ItemType])
GO

-- Running this query with Include Actual execution plan.
SELECT 
 id.ItemAutiId,
 id.ItemId,
 id.ItemName,
 id.ItemType,
 id.ItemType,
 id.ItemDescription
FROM ItemDetails id
WHERE id.ItemType = 'PK'
GO

 
--Now creating a clustered index on table
CREATE CLUSTERED INDEX [IX_ItemAutiId]
ON [dbo].[ItemDetails] ([ItemAutiId])
GO

-- Again running this query with Include Actual execution plan after creating clustered index.
SELECT 
 id.ItemAutiId,
 id.ItemId,
 id.ItemName,
 id.ItemType,
 id.ItemType,
 id.ItemDescription
FROM ItemDetails id
WHERE id.ItemType = 'PK'
GO

You can see here first case did table scan and second case did clustered index scan. Now moving other shots,

2. Covering nonclustered index behavior without and with clustered index on table.

-- Dropping existing indexes on table
DROP INDEX [IX_ItemType] ON  [ItemDetails]
DROP INDEX [IX_ItemAutiId] ON  [ItemDetails]

GO

-- Creating covering index on table
CREATE NONCLUSTERED INDEX [IX_ItemType]
ON [dbo].[ItemDetails] ([ItemType])
INCLUDE ([ItemId],[ItemName],[ItemDescription])
GO

-- Running this query with Include Actual execution plan.
SELECT 
 id.ItemAutiId,
 id.ItemId,
 id.ItemName,
 id.ItemType,
 id.ItemType,
 id.ItemDescription
FROM ItemDetails id
WHERE id.ItemType = 'PK'
GO

-- Creating clustered index on table
CREATE CLUSTERED INDEX [IX_ItemAutiId]
ON [dbo].[ItemDetails] ([ItemAutiId])
GO

-- Again running this query with Include Actual execution plan after creating clustred index.
SELECT 
 id.ItemAutiId,
 id.ItemId,
 id.ItemName,
 id.ItemType,
 id.ItemType,
 id.ItemDescription
FROM ItemDetails id
WHERE id.ItemType = 'PK'
GO

In this scenario first case did table scan and second case did non-clustered index seek. Do you have any more idea about it if you experienced with it?