Question

SQL Server - Clustered and NonClustered index on the same column?!?

Asked by: Endelm

Hi,

I saw a table in my database that has custered and nonclustered index on the same column!

Does this make sense? Isn't it a waste of time to have the nonclustered index there, since its on the exacly same column as the primary index?

There is no gain in having the nonclustered index, right? Should be deleted?

Thank you! :)

ALTER TABLE dbo.MyTable ADD  CONSTRAINT PRIMARY KEY CLUSTERED 
(
	[id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 70) ON [PRIMARY]
 
 
 
ALTER TABLE dbo.MyTable ADD  CONSTRAINT UNIQUE NONCLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 70) ON [PRIMARY]

                                  
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Asked On
2009-10-30 at 04:39:23ID24857694
Topics

SQL Query Syntax

,

SQL Server 2005

,

SQL Server 2008

Participating Experts
3
Points
500
Comments
9

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Answers

 

by: brejkPosted on 2009-10-30 at 04:45:10ID: 25701930

Drop the second (nonclustered) index. It's useless.

 

by: rrjegan17Posted on 2009-10-30 at 04:45:29ID: 25701933

>> Does this make sense?

No.. A clustered index is far better than Non-clustered index.
And someone would have created it by mistake.

If this is the case for all your tables, then delete these redundant index.

>> Isn't it a waste of time to have the nonclustered index there, since its on the exacly same column as the primary index?

Yes.. Its a waste of resources.

>> There is no gain in having the nonclustered index, right? Should be deleted?

Yes.. And it needs to be deleted.

 

by: brejkPosted on 2009-10-30 at 04:49:04ID: 25701956

@Endelm: I can imagine only one situation when the nonclustered index would be useful. When the most often performed query is COUNT(*) on the table.

 

by: QlemoPosted on 2009-10-30 at 04:50:45ID: 25701967

I do not agree. There are occasions where an additional non-clustered index can make sense. E.g. each time a query can be completely resolved by using only the non-clustered index, which is much more compact and hence efficient than reading the clustered index (which is in fact the whole table).

That said, in most cases such an index is created by mistake.

 

by: rrjegan17Posted on 2009-10-30 at 04:51:31ID: 25701973

>> When the most often performed query is COUNT(*) on the table.

But that can be converted to count(1) for better performance.

 

by: EndelmPosted on 2009-10-30 at 04:53:09ID: 31647945

Thanks guys, I thought so :)

 

by: brejkPosted on 2009-10-30 at 04:57:28ID: 25702015

@rrjegan17: But there has to be an index chosen for this kind of operation anyway. And the nonclustered index on id would seem to be quite narrow and can be potentially chosen by the optimizer.

 

by: QlemoPosted on 2009-10-30 at 04:58:01ID: 25702019

This is a myth, and might have been true some time ago. But not with MSSQL 2005 and higher. Count(1) does not perform better than count(*). The optimizer is able to reduce both to the necessary data access path.

 

by: brejkPosted on 2009-10-30 at 05:03:28ID: 25702058

@Qlemo: That's what I expected ;-) Just the same myth as IF EXISTS (SELECT *...) and IF EXISTS (SELECT 1 ...). Good point, man!

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