When SQL Server receives a new query, the Query Optimizer undertakes the task of identifying a good plan to resolve the query. It considers a number of different factors as it analyzes the query and maps out a way in which to retrieve the information requested. Dr. DeWitt did an amazing job of explaining this process in understandable terms at the 2010 PASS Community Summit.

Whether or not the query optimizer deems an index to be useful in resolving a query largely depends on the information contained in the statistics for that index.

If the statistics are outdated and do not accurately represent the distribution of values in the table, the query optimizer may not produce an optimal plan for resolving the query. Misleading statistics may result in the optimizer not using an index when it should, or using an index when it would be more efficient to scan the table. Statistics also influence the logical join order and physical types selected.

That’s why it is crucial that the statistics be updated regularly. How often? Well, that, of course, is going to depend on the data being stored, the frequency of updates, inserts, and deletes to the table, etc. It could be nightly; it could monthly. It just depends.

So, how can you tell when the statistics where last updated for an index?

The following query demonstrates this in SQL Server 2005/2008. It makes use of the sys.indexes and sys.tables catalog views, along with the STATS_DATE() function, to retrieve the date that each index was last updated for every user table in the current database.

SELECT

   t.name AS Table_Name

   ,i.name AS Index_Name

   ,i.type_desc AS Index_Type

   ,STATS_DATE(i.object_id,i.index_id) AS Date_Updated

FROM

   sys.indexes i JOIN

   sys.tables t ON t.object_id = i.object_id

WHERE

   i.type > 0

ORDER BY

   t.name ASC

   ,i.type_desc ASC

   ,i.name ASC

In SQL Server 2000, a similar query can be run.

SELECT

   o.name AS Table_Name

   ,i.name AS Index_Name

   ,STATS_DATE(o.id,i.indid) AS Date_Updated

FROM

   sysobjects o JOIN

   sysindexes i ON i.id = o.id

WHERE

   xtype = ‘U’ AND

   i.name IS NOT NULL

ORDER BY

   o.name ASC

   ,i.name ASC

These queries can be useful while troubleshooting and diagnosing performance-related issues. Sometimes, it’s as simple as outdated statistics.

Filed under: Administration, Optimization, SQLServerPedia Syndication, T-SQL