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_WA_SYS statistics created - should you index on these as well

Paul_Blackler
on
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Last Modified: 2008-01-09
We have a number of WA_SYS statistics created (auto on) against tables. Is it wise to create indexes for these fields listed in the WA_SYS if you dont have them setup already. This is a DSS environment.

Thnaks

Paul
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Kevin HillSr. SQL Server DBA
Commented:
I wouldn't blindly go create an index just because the system did it.  I would rather do some investigating and (SQL) profiling to see where my perf can be improved and create indexes there if necessary.

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Be careful - statistics are used by the optimizer to choose the path for the query - they are not actually used to execute the query itself.  

A lot of small, low cardinality columns will be scanned by the optimizer and stats will be created. You do not want indexes on these columns because their low cardinality (low number of values) means an index will not help much (imagine a table of states that also includes the country but there is only one value for country - indexing country does not help your query performance).

Check the strategies and base your indexes on what the strategies are doing. If you see time consuming tablescans, definitely create indexes to prevent that from happening.
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