Dave Ballantyne Clear Sky SQL

›Freelance Database Developer/Designer –Specializing in SQL Server for 15+ years ›SQLLunch –Lunchtime usergroup –London & Cardiff, 2 nd & 4 th Tuesdays ›TSQL Smells script author – › ›Twitter

›This is also me –Far to often …. ›Estimates are central ›Statistics provide estimates

›Every Journey starts with a plan ›Is this the ‘best’ way to Lyon? –Fastest –Most efficient –Shortest ›SQL Server make similar choices

›SQL Server is a cost based optimizer –Cost based = compares predicted costs ›Therefore estimations are needed ›Every choice is based on these estimations ›Has to form a plan –And the plan cannot change in execution if ‘wrong’

›Costs are not actual costs –Little or no relevance to the execution costs ›Cost is not a metric –1 <> 1 anything ›Their purpose: –Pick between different candidate plans for a query

›Included within a index ›Auto Updated and created –Optimizer decides “It would be useful if I knew..” –Only on single column –Not in read-only databases ›Can be manually updated ›Auto-Creation can operate Async

›DBCC SHOW_STATISTICS(tbl,stat) –WITH STAT_HEADER ›Display statistics header information –WITH HISTOGRAM ›Display detailed step information –WITH DENSITY_VECTORS ›Display only density of columns – Density = rows / count of distinct values – WITH STATS_STREAM ›Binary stats blob ( not supported )

Total rows in table Rows read and sampled No of steps in histogram – Max 200 Density (Rows/Distinct values) (exc boundaries) not used Avg byte len for all columns

 Each step contains data on a range of values  Range is defined by ◦ <= RANGE_HI_KEY ◦ > Previous range RANGE_HI_KEY Row 3 > 2 and <=407 Row 3 > 2 and <=407 Row 4 > 407 and <=470 Row 4 > 407 and <=470 6 rows of data = Rows of data > 407 and < Rows of data > 407 and < Distinct values > 407 and < Distinct values > 407 and < 470 Density (17 / 9)

RANGE_HI_KEY <= Predicate AND > Previous RANGE_HI_KEY RANGE_HI_KEY <= Predicate AND > Previous RANGE_HI_KEY As Predicate == RANGE_HI_KEY Estimate = EQ_ROWS As Predicate == RANGE_HI_KEY Estimate = EQ_ROWS

RANGE_HI_KEY <= Predicate And > previous RANGE_HI_KEY RANGE_HI_KEY <= Predicate And > previous RANGE_HI_KEY As Predicate < RANGE_HI_KEY Estimate = AVG_RANGE_ROWS As Predicate < RANGE_HI_KEY Estimate = AVG_RANGE_ROWS

›Greater accuracy on Range boundary values –Based upon the ‘maxdiff’ algorithm ›Relevant for leading column –Estimate for Smiths –But not Smiths called John ›Additional Columns cumulative density only –1/(Count of Distinct Values)

Density vector = 1/(Count of Distinct Values) =19,517 1 / 19,517 = ~ E-05 =19,517 1 / 19,517 = ~ E-05

211 * E-05 = ~ * E-05 = ~

›All Diazs will estimate to the same: –As will all Smiths,Jones & Ballantynes –The statistics do not contain detail on FirstName –Only how many distinct values there are –And assumes these are evenly distributed ›Not only across a single Surname ›But ALL Surnames

›So far we have only used a single statistic for estimations ›For this query: ›To provide the best estimate the optimizer ideally needs to know about LastName and FirstName

›Correlating Multiple Stats ›AND conditions ›Find the intersection LastName = ‘Sanchez’ FirstName = ‘Ken’

›And logic –Intersection Est = ( Density 1 * Density 2) ›Or logic –Row Est 1 + Row Est 2 –(Intersection Estimate) –Avoid double counting

10% * 10 % = 1% 10% * 20 % = 2% No Correlation in the data is assumed

To keep statistics fresh they get ‘aged’ 0 to > 0 <= 6 Rows (For Temp Tables) 6 Modification <= 500 Rows 500 Modifications >= 501 Rows % of table Will cause statistics to be updated on next use Will cause statements to be recompiled on next execution Temp tables in stored procedures more complexcomplex

 Trace flag Dynamically lower statistics update threshold for large tables  >25,000 rows  2008r2 (SP1) & 2012

›When density vector is not accurate enough ›Manually created statistics only ›Additional where clause can be utilised Filter Expression = Filter Unfiltered Rows = Total rows in table before filter Filter Expression = Filter Unfiltered Rows = Total rows in table before filter Rows Sampled Number of filtered rows sampled Rows Sampled Number of filtered rows sampled

Density of London * Density of Ramos Filter is matched and histogram is used

›For ‘large’ data sets a smaller sample can be used ›Here 100% of the rows have been sampled ›Here ~52% of the rows have been sampled ›Statistics will assume the same distribution of values through the entire dataset

›Also Auto/Forced Parameterization

›Remember the Density Vector ? ›19972 (Total Rows )* = › On Equality The Average Density Is Assumed

›Stored Procedures

›Enables a better plan to be built –(most of the time) –Uses specific values rather than average values ›Values can be seen in properties pane ›Erratic execution costs are often Parameter Sniffing problems

›Force a value to be used in optimization ›A literal value ›Or UNKNOWN –Falls back to density information

›OPTION(RECOMPILE) –Recompile on every execution ›Because the plans aren’t cached –No point as by definition the plan wont be reused ›Uses variables as if literals –More accurate estimates

›The Achilles heel –A plan is fixed ›But… The facts are: –More Smiths than Ballantynes –More Customers in London than Leeds LondonLeeds Smith50050 Ballantyne101

›This is known as the ‘Plan Skyline’ problem

›But wait… ›It gets worse –That was only EQ_ROWS –RANGE_ROWS ??

›Variations in plans –Shape ›Which is the ‘primary’ table ? –Physical Joins –Index (non)usage ›Bookmark lookups –Memory grants ›500 rows need more memory than 50 –Parallel plans

›Can the engine resolve this ? –No! ›We can help though –And without recompiling –Aim is to prevent ‘car-crash’ queries –Not necessarily provide a ‘perfect’ plan Demo

›“There is always a trace flag” –Paul White ›TF 9292 –Show when a statistic header is read ›TF 9204 –Show when statistics have been fully loaded ›TF 8666 –Display internal debugging info in QP

›Statistics Used by the Query Optimizer in Microsoft SQL Server 2008Statistics Used by the Query Optimizer in Microsoft SQL Server 2008 ›Plan Caching in SQL Server 2008Plan Caching in SQL Server 2008 ›SQL Server internals 2008 book (MSPress)

›My ›Twitter