1. Statistics: What are they and How do I use them
Dave Bland
@SQLDave29

2. Thank you to our Sponsors

3. About Me 14 years DBA Experience 10 Years Application\BI development
18 years of teaching SQL Server
5 years SQL Server Instructor at Harper College, Palatine, IL
Currently supervises the Shared Services DBA team for Stericycle
Frequent Presenter at SQL Saturday event throughout the Midwest

4. Certifications

5. Agenda What are Statistics How are they created
How does the SQL Server use them
Get information about statistics
Statistics at Work

6. Statistics Not at Work

7. What are Statistics Also known as Distribution Statistics Cardinality
Refers to the uniqueness data values contained in a particular column
Used by the Query Optimizer
Is a Cost based optimizer
You don't need to know about them to execute queries, but the better you understand them, the better you can optimize your queries and sort out performance problems
--Robert Sheldon, Redgate Web site

8. Factors for Cost Based Optimization
Availability of indexes
Availability of Statistics
CPU Costs
I/O Costs
Cost of the Plan is a sum of the above
If statistics are missing, could pick a suboptimal plan

9. Statistics Basics Statistics Objects are BLOB objects
Hold Cardinality Estimates
SQL Server 2014 and 2016 include rewritten Cardinality Estimator logic
Query Optimizer could use cardinality estimates to choose the index seek operator instead of the more resource-intensive index scan operator
Types
Single Column
Multi-Column
Filtered Statistics

10. Why are Statistics Important
Can help improve performance
Unless:
Out-of-Date statistics
Missing Statistics
Functions in Predicates
Table variables

11. Ways to Create Statistics
Statistics created due to index creation. These statistics have the index name
Statistics created by Optimizer(Column statistics).
_WA_sys_<<<COLUMN>>>>_<<<<ObjectID Hexidecimal>>>>
User defined statistics which are created with CREATE STATISTICS command by the DBA

12. When are Statistics Created
Can be single column or multi-column
Cannot be created on a Read-Only database
Auto-Create always creates single column statistics
When a column is used in a JOIN or WHERE
When Auto Create Statistics is turned on
SELECT LastName ,
modifieddate
FROM [Person].[Person]
WHERE LastName = ‘Smith’
Demo # 1

13. Drop Statistics
Be careful when you drop statistics. Doing so may affect the execution plan chosen by the query optimizer.
Statistics on indexes cannot be dropped by using DROP STATISTICS. Statistics remain as long as the index exists.
Requires Alter Table rights

14. Filtered Statistics Built on a subset of rows in a table
Can be useful on large tables
Supports only simple comparison operators
<, > ,<= ,>= , =, IS NULL and IS NOT NULL
Cannot be created on computed columns
Cannot be created on User Defined Datatypes
Cannot be created on Indexed Views

15. Updating Statistics Should be part of any solid maintenance strategy
Synchronous (the default) vs Asynchronous
Synchronous
AUTO_UPDATE_STATISTICS_ASYNC option
TRUE or FALSE
Faster plan creation
Create the plan before updating stats

16. Update Statistics Requires queries to be recompiled Auto Update
Default threshold is 20% + 500
Requires ALTER permissions
Rebuild Indexes
Examples
EXEC sp_updatestats;
update statistics t1 (a) with sample 10 rows;
UPDATE STATISTICS Production.Product([StatsProductName])
WITH FULLSCAN
UPDATE STATISTICS Customer;
Demo – #2A

17. sp_autostats Can be used to disable auto update stats for a column
Turn auto update stats for an entire table
View status of statistics
EXEC sp_autostats 'Production.Product', 'ON'; GO
EXEC sp_autostats 'Production.Product', 'OFF', AK_Product_Name;
EXEC sp_autostats 'Production.Product';
Demo – #3

18. Viewing Statistics DBCC SHOW_STATISTICS Sys.stats In Management studio
Header
Density vector
Histogram
Sys.stats
In Management studio
Demo – #5

19. How to Find Last Date Updated
DBCC SHOW_STATISTICS
STATS_DATE Function
SELECT OBJECT_NAME(object_id) AS [ObjectName]
      ,[name] AS [StatisticName]
      ,STATS_DATE([object_id], [stats_id]) AS [StatisticUpdateDate]
FROM sys.stats;

20. Related Trace Flags 2371 2453 Started with SQL Server 2008 R2
SQL Server 2014 or older off by default
SQL Server 2016 on by default
Can be used to lower the threshold
2453
Started with SQL Server 2012 SP2
Can speed up when using table variables
Can also use: OPTION(RECOMPILE)

21. Effects of Trace Flag 2371
One Million Rows would update at 200,500(20% + 500)
Full Scan

22. Extended Events and Execution Plans
Important to know when queries run with missing statistics
Demo – #4

23. Single Column vs Multi-Column
Cardinality estimates become more difficult when there is more than one predicate
SQL Server will not automatically create multi-column statistics
For Multi-Column, Histograms are only created on the first column
Once created, if Auto Update Statistics is turn on – SQL Server will keep them updated
Demo – Multi Column

24. How do we know statistics are being used?
One good check you can do is when you generate execution plans for your queries:
Check out your “Actual Number of Rows” and “Estimated Number of Rows”.
If these numbers are (consistently) fairly close, then most likely your statistics are up-to-date and used by the optimizer for the query. If not, time for you to re-check your statistics create/update frequency.
Index scans on indexes that are not fragmented and the statistics are up to date
See Next Slide

25. Statistics In Use or Are They?
No Statistics
Statistics

26. Export Statistics

27. Best Practices Create appropriate indexes
Review Indexes from time to time
Be Lazy!!!!!
Keep statistics up to date
Use a computed column or a temporary table
To replace a table variable
Consider using OPTION(RECOMPILE)
Forces the actual row count of the table variable
Consider enabling Trace Flag 2453 if using table variables
Review to see if Trace Flag 2371 might help, Off be default on pre-2016 servers
Consider using a combination of automatically updated Statistics and manually updated Statistics
Review execution plans of poor performing queries
Implement an Extended Events session to find missing statistics

28. Resources

29. Thank You!!!!