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Why SQL Server is not using non-clustered index


Why SQL Server is not using non-clustered index

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vigneshbeece
vigneshbeece
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In SQL server 2008R2, Table having Non-clustered index and table contains 2 crore records, while executing "Select count(*) from table name" it going for table scan, its not using non clustered index. Why index is not used?
GilaMonster
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It's going to have to scan something, since there's no filter on that query. Optimiser calculated that a scan of the table will be the most efficient.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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vigneshbeece
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If any where condition is there, optimizer would use Non-clustered index right?
GilaMonster
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Maybe. Depends on the predicate, the index key columns and the rest of the query. It's not as simple as you're making out.
There's a lot of posts on indexes and index usage on my blog. https://www.sqlinthewild.co.za/index.php/category/sql-server/indexes/

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
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Arsh
Arsh
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Since it does cost based optimization , based on 'estimates' SQL decides to go with the least costly plan. Depending upon , say , how stale are the stats for example , the optimizer will prefer doing a scan if its cheaper (at plan level) than using the index .

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