1. Query Optimization Statistics: The Driving Force Behind Good Performance
G. Vern Rabe -

3. Vern Rabe
Portland, OR independent consultant, trainer MCTA, MCDBA, MCSE, MCITP Oregon PASS Chapter Leader Working with/focusing on SQL Server since 1993, starting with SQL Server 4.21a My first PC had a 10MB hard drive
G. Vern Rabe –

4. The Optimizer Determines how to execute a query
Simple 2 table JOIN example •
3 join operators
Order of tables
Filter/predicate …
Cardinality major factor
Cardinality determined from statistics
Cardinality estimator rewritten in SQL Server 2014 4
G. Vern Rabe –

5. What are Statistics?
Estimated summary information of distribution of values in one or more columns
All indexes
Any column used in predicate (by default)
Any column(s) you define
Header, density vector, histogram 5
G. Vern Rabe –

6. How Do I Read Statistics?
SSMS •
sp_helpstats – deprecated
DBCC SHOW_STATISTICS •
WITH STAT_HEADER
WITH DENSITY_VECTOR
WITH HISTOGRAM
sp_autostats •
sys.stats
sys.stats_columns
sys.dm_db_stats_properties • 6
G. Vern Rabe –

7. How Should Statistics be Maintained?
AUTO_UPDATE_STATISTICS[_ASYNC]
SQL Server < 2014 – for tables > 500 rows, 20% row changes
SQL Server >= 2014 – “more aggressive” as table rows increase
sp_autostats - turn off/on for table or statistic object
AUTO_CREATE_STATISTICS •
UPDATE STATISTICS •
WITH FULLSCAN
WITH <number> PERCENT | ROWS
If you manually update stats, it’s recommended to disable auto stats for that statistics object, to avoid overwriting high sample size with lower “standard” sample size
sp_createstats – creates column statistics where they don’t already exist
sp_updatestats – executes UPDATE STATISTICS for entire database 7
G. Vern Rabe –

8. What Can Go Wrong? Bad Sample Ascending Key
Statistics are estimates – not exact
By default, only reads SOME of the data •
Ascending Key
Statistics Outliers •
<= 2012, cardinality estimate of 1
>= 2014, cardinality estimate of 30% of rows changed
sp_FindStatisticsOutliers • 8
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9. Summary
Statistics are über important for performance Multiple ways to view statistics Statistics are great but not perfect Ascending Key issues – override default updating of statistics 9
G. Vern Rabe –

11. Vern Rabe Thanks. Questions? 11 G. Vern Rabe – vern@rabedata.com