unique versus non-unique index in terms of performance

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unique versus non-unique index in terms of performance

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Mani-989491

Posted Friday, February 06, 2009 6:29 AM

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In terms of performance which one should be created between unique and non-unique index. Let us say I want to make a Non clustered index on columns which is idenetity column. As I alredy know this columns value is unique so if I create a non-unique index will that have any significant performance over the unique index.

My concern is as unique index will check for any duplicate existing row for every new entry and I have some 70000 records added to the table every day so will my upload into the table will take more time after I create this unique index. Also which index will have better query performance.

Post #651562

Grant Fritchey

Posted Friday, February 06, 2009 8:08 AM

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It depends on the queries being run. The optimizer will take into account when an index is unique and it can improve performance, but it really does depend on the query. It might not affect things at all.

It will slow down inserts slightly, but probably not enough to notice. It doesn't have to check everything single record against every other record. It's going to use the index itself for the check which will be quite fast.

In general, if you've determined that an index will help and the data being indexed is going to be unique, I'd say go ahead and make it a unique index. It won't cost much and you will receive benefits.

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"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood..." Theodore Roosevelt
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Post #651695

Mani-989491

Posted Friday, February 06, 2009 8:49 AM

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Thanks Grant

Post #651754

Ninja's_RGR'us

Posted Friday, February 06, 2009 9:04 AM

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I'd go the complete opposite. If the data requiresa unique index (for data quality), then go ahead and create the index. I wouldn't do a unique index just because it might go faster and not consider wheter or not it could bug the applications using it.

Post #651767

Grant Fritchey

Posted Friday, February 06, 2009 9:08 AM

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Oh no, if it's going to bug the apps, then clearly the data is not guaranteed unique. I'm just saying if the data is going to be unique, there's no reason to not make the index unique and several reasons to make it that way.

----------------------------------------------------
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood..." Theodore Roosevelt
The Scary DBA
Author of: SQL Server 2012 Query Performance Tuning
SQL Server 2008 Query Performance Tuning Distilled
and
SQL Server Execution Plans

Product Evangelist for Red Gate Software

Post #651774

Vijaya Kadiyala

Posted Monday, February 09, 2009 9:05 AM

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I Agree with Ninja's decision. If the data requires to have Unique Index then we should have Unique Index. In this world anything can happen with Data Quality. So always impose contraints on this to avoid any future Data Quality Issues.

Thanks -- Vijaya Kadiyala
http://dotnetvj.blogspot.com

Thanks -- Vijaya Kadiyala
www.dotnetvj.com
SQL Server Articles For Beginers

Post #652904

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