1. SQL Server Statistics 101
Travis Whitley Senior Consultant, Oakwood Systems Group
@whitleysql
whitleysql.wordpress.com

2. Presentation Roadmap Foundation Demo: Visualizing Statistics
The Big Picture – Query Processing
Statistics Primer
Demo: Visualizing Statistics
The Statistics Object
Cardinality Estimation
Problems and Solutions
Best Practices
Maintenance Recommendations

3. The Big Picture
Statistics Primer
Foundation

4. The Big Picture – Query processing
Think about data access
Selectivity
Which query do you think will benefit most from an index on ProductType?
select * from Transactions
where ProductType = ‘Fuel'
where ProductType = ‘Food'
where ProductType = ‘Toiletries‘
-So let’s think about data access. Gas station transactions table example.

5. The Big Picture – Query processing

6. Statistics Primer
What are statistics?
Statistics contain a map of the data distribution for a column or index
Statistics are real database objects
The statistics object is stored in the sysidxstats table in the primary filegroup and in a binary large object or statblob (internal table)
They contain density information and a histogram for the first column in the statistics object (high order element of data set)
-Size
30,000 stats per table in SQL 2008 R2
- Transition from this slide to the next slide

7. Statistics Primer Creating Statistics Updating Statistics
AUTO_CREATE_STATISTICS
Set to ON by default
Will create SINGLE column statistics based on query WHERE/JOIN
Never MULTI-column
Index creation
Statistics always created along with new Index
SINGLE or MULTI-column
CREATE STATISTICS statement
Manual creation for given column(s)
sp_createstats stored proc
Will create statistics on all columns for all user tables in the current database, except for XML columns
Excludes columns with existing statistics
AUTO_UPDATE_STATISTICS
Set to ON by default
Colmodctrs
SAMPLED based on default sample rate
sp_updatestats stored proc
Updates all existing stats, if necessary
Uses old rowmodctr, not colmodctr
UPDATE STATISTICS statement
Allows specific updates of distinct stats
Can specify sample size or FULLSCAN
Can specify all (default), columns, or index
INDEX REBUILD
Will update index linked statistics, equivalent of FULLSCAN
Will NOT update auto generated or column statistics
Statistics Primer

8. When SQL Server “takes care of itself”
Statistics Primer
When SQL Server “takes care of itself”
The default sample rate gets relatively lower as tables grow
Autoupdate is only triggered if the colmodctrs ‘stale’ threshold is reached AND the relative index/column is queried
Manual or automatic updating invalidates any cached plan utilizing those statistics
Index Reorganization/DEFRAG will NOT update index linked statistics
Autocreate will not work on Read Only databases
Autoupdate is generally beneficial, but in certain scenarios it can be problematic

9. DEMO: Visualizing statistics
The Statistics Object
Cardinality Estimation
DEMO: Visualizing statistics

10. Problems and solutions
Best Practices
Maintenance Recommendations
Problems and solutions

11. Keep AUTO_CREATE_STATISTICS ON by default
Best Practices
Keep AUTO_CREATE_STATISTICS ON by default
Keep AUTO_UPDATE_STATISTICS ON by default
Try AUTO_UPDATE_STATISTICS_ASYNC for large OLTP tables if auto updating is an issue
Recommend stored procs with parameters rather than local variables in queries
Create filtered statistics when 200 histogram steps won’t suffice
Eliminate any duplicate statistics
If you notice estimate vs actual are off while troubleshooting, try updating the statistics

12. Maintenance Recommendations
Schedule an Update statistics job with large sample or FULLSCAN if feasible
Nightly for larger/OLTP tables
Weekly for smaller, less updated tables
Update COLUMN statistics only on INDEX REBUILD days
Stats on tables with ascending key columns may need updated more frequently than other tables
Automate script that will pin point out of date or auto updated stats

13. Holger Schmeling: SQL Server Statistics SQL Server Internals 2008
References
Books
Holger Schmeling: SQL Server Statistics
SQL Server Internals 2008
Online
BOL: SQL Server Statistics
Fabiano Amorim Blog
Glenn A Berry Blog