Indexes Out the Window

Categories: News, Professional
Comments: 1 Comment
Published on: January 22, 2010

I ran into a peculiarly puzzling situation with a query I have been tuning of late.  The query runs some aggregations and performs a range scan based on a date value passed to the query.  The primary target for this query is to be consumed by reporting services.  The query executes (from a stored proc) three times in a single report.  This is another case were the report currently outperforms the previous version by several fold.  I just want it to run a little faster – it is an executive report afterall.

While tuning the query itself, I removed some of the aggregations (not consumed by the report and not needed for accuracy), and that bought me a small performance gain.  Next was to remove a couple of “or” operations.  Again, that only bought me a small performance gain.

The next step was to check my indexing.  Looking at the maintenance plan for the query, indexes were being used – or so it seemed.  Based on the execution plan, the query was performing two Index Scans prior to a Hash Match.  In the first Index Scan, the query was expecting an Output of two columns (SalesOppID and LastUpdate) from IDX_SalesOpportunity_SalesStatusIDSalesContactID.  This is depicted in the next two images.

This makes a little sense since the index does contain the LastUpdate column.  But the index is not covering, and we do not have a Key lookup.

[codesyntax lang="sql" title="IDX_SalesOpportunity_SalesStatusIDSalesContactID"]

[/codesyntax]

Out of curiosity, I decided to see what would happen if I commented out the lines that required the LastUpdate field.  I ended up with the following:

Interesting, the query optimizer decided that we should use IDX_LeadTypeID which has the following definition:

[codesyntax lang="sql" title="IDX_LeadTypeID"]

[/codesyntax]

Once again, I have a query that is using a bad index.  There is no key lookup and the index is not covering.  I find this very peculiar.  Thus I proceeded to test a little more.  I dropped that Index and reran the query.  With each group of tests, it is important to note, I cleared the ProcCache.  After dropping the index, my execution plan showed no performance increase and still showed a funny result.  The query moved onto a new index that was not covering and there was no key lookup.

Now, let’s make this a little more interesting.  I do have a Clustered Index on SalesOppID – defined as follows:

[codesyntax lang="sql" title="PK_SalesOpportunity"]

[/codesyntax]

I am puzzled as to why my query will not use the Clustered Index.  I decided to do the standard stuff – DBCC CheckDB, DBCC UpdateUsage etc.  There is no corruption, and the query still tries to perform the same way.  I could recreate the clustered index on the LastUpdate field.  Since there are Foreign Key relationships that I do not want to drop and recreate, I will just create an NC index on the appropriate two fields.  After doing that, I get the following:

The index is covering and pulling the correct columns.  With this index the query is about 3 times faster than previously.  Some of that is due to the change from a Hash Match to a Merge Join from the two tables (there is still a hash match but it is after the merge and is now less costly).

The moral is, even though an index may be used by the query – it may not be a good index choice.  I am still working on tuning this query, I think I can squeeze better performance out of it.

Execution Plans can be downloaded here.

1 Comment - Leave a comment
  1. Dugi says:

    Not bad explanation and very nice tuning logic!

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