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Adding Indexes to Table hurts query performance (self-join)

Posted on 2007-11-26
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Last Modified: 2012-06-27
I'm optimizing a query on SQL 2000.  The query joins a single table to itself 5 times to get various MAX(values).  

When I add a large covering clustered (or non-clustered index) to the table to help the query, performance suffers greatly.  This is a select statement only.

Why would a large index negatively impact read performance ?
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Question by:JaffaKREE
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LVL 16

Expert Comment

by:SQL_SERVER_DBA
ID: 20352377
DO REBUILD
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Author Comment

by:JaffaKREE
ID: 20352448
Rebuild the index ?  I've just created it.
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JR2003 earned 2000 total points
ID: 20352475
How many rows has the table?
Are you accessing columns that aren't in the index?
Can you paste the query in with the indexes you have created?
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by:JaffaKREE
ID: 20352547
> Are you accessing columns that aren't in the index?

After looking at this, I believe I made a mistake and excluded a necessary column from the index.  

I was concerned that the process, since it was accessing the same table/index 5 times, may have been deadlocking/blocking itself.


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Expert Comment

by:JR2003
ID: 20353758
sometimes it does take longer using an index depending one how many rows are accessed and what columns are on the index.
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