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Quering sys.dm_db_index_physical_stats and sys.indexes has a negative impact on other queries running Expand / Collapse
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Posted Wednesday, November 05, 2008 3:15 AM
Grasshopper

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I used DMV's to get some detailed information on indexes. Running the query resulted in significant overall performance degradation. Stopping the query immediately solved performance issues.

The query used the following join construction:

sys.dm_db_index_physical_stats
(8
,NULL -- NULL to view all tables
,NULL -- NULL to view all indexes; otherwise, input index number
,NULL -- NULL to view all partitions of an index
,'DETAILED') --We want all information
AS a
INNER JOIN sys.indexes
ON a.[object_id] = sys.indexes.[object_id]
AND a.index_id = sys.indexes.index_id

The question is:
How can this query impact other queries in such a negative way?

Kind regards,
Post #597169
Posted Wednesday, November 05, 2008 3:53 AM


SSCertifiable

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Because of the parameter "DETAILED", it will perform like a dbcc updateusage with countrows....

BOL states:
.... The nonleaf levels of indexes are only processed when mode = DETAILED.
...
The DETAILED mode scans all pages and returns all statistics
...


So you may experience IO overhead, locking ( ? ) , ...


Johan


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Post #597198
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