1. Maciej Pilecki | Project Botticelli Ltd.

2. SELECT Bio FROM Speakers WHERE FullName=‘Maciej Pilecki’;  Microsoft Certified Trainer since 2001  SQL Server MVP since Jan 2006  Specializing in SQL Server database development and administration  Delivering training and consulting services around the world  Frequent speaker at many international conferences and UG meetings

3. Agenda  Introduction to statistics  Statistics internals  Creating statistics  Maintaining statistics

4. How the Query Optimizer Works Source: SQL 2008 Books Online

5. How the Query Optimizer Works  SQL Server uses "cost-based" optimizer  Execution plan is chosen based on the ESTIMATED cost of execution  Cost estimation based on the number of rows processed (cardinality estimation)  Cardinality estimation based on ○ Number of rows in a table ○ Data distribution statistics  Optimizer estimates selectivity of a query

6. What are Statistics?  Statistics hold data distribution information about of values in a column or a set of columns  Statistics contain: Distribution for leading column Density information for column combinations  Every index has associated statistics

7. Statistics internals  Viewing statistics: sys.stats sys.stats_columns DBCC SHOW_STATISTICS

8. Creating statistics  Automatically: Depends on AUTO_CREATE_STATISTICS database option For every index For each non-indexed column used in the query predicate  Manually: CREATE STATISTICS sp_createstats

9. Maintaining statistics  Outdated stats can lead to suboptimal execution plans  Stats have to be refreshed

10. Maintaining statistics  Automatically, depending on: ○ AUTO_UPDATE_STATISTICS database setting ○ STATISTICS_NORECOMPUTE option of the index ○ NORECOMPUTE option of the statistics ○ sp_autostats ○ AUTO_UPDATE_STATISTICS_ASYNC database option  Manually: ○ UPDATE STATISTICS ○ sp_updatestats ○ Index rebuild

11. Auto-updates  Thresholds for auto-updates: For temp table with less than 6 rows – after every 6 updates For table with less than 500 rows - after 500 updates For larger table - after 500 + 20% of rows For table variables – NEVER Described in KB195565 Updates tracked in rowmodctrs sysindexes

12. Best practices  Don't turn off: AUTO_CREATE_STATISTICS AUTO_UPDATE_STATISTICS  Rebuild your indexes from time to time Performs a full-scan update of statistics  Run sp_updatestats after any major update

13. Summary  Introduction to statistics  Statistics internals  Creating statistics  Maintaining statistics

14. Related Content  Statistics Used by the Query Optimizer in Microsoft SQL Server 2008 http://msdn.microsoft.com/en-us/library/dd535534.aspx  Elisabeth Redei – Lies, Damn Lies and Statistics (in two parts): http://sqlblog.com/blogs/elisabeth_redei/archive/2009/03/01/lies- damned-lies-and-statistics-part-i.aspx  SQL Server Internals and Query Tuning class: Mainz, 22-26 November http://entwickler-akademie.de/codecamps/workshop-maciej-pilecki