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

Posted on 2007-11-26
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 ?
Question by:JaffaKREE
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LVL 16

Expert Comment

ID: 20352377

Author Comment

ID: 20352448
Rebuild the index ?  I've just created it.
LVL 18

Accepted Solution

JR2003 earned 500 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?

Author Comment

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.

LVL 18

Expert Comment

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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