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Special Topics

Special Topics. Joe Chang www.qdpma.com. About Joe. SQL Server consultant since 1999 Query Optimizer execution plan cost formulas (2002) True cost structure of SQL plan operations (2003?) Database with distribution statistics only, no data 2004 Decoding statblob/stats_stream

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Special Topics

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  1. Special Topics Joe Chang www.qdpma.com

  2. About Joe • SQL Server consultant since 1999 • Query Optimizer execution plan cost formulas (2002) • True cost structure of SQL plan operations (2003?) • Database with distribution statistics only, no data 2004 • Decoding statblob/stats_stream • writing your own statistics • Disk IO cost structure • Tools for system monitoring, execution plan analysis See ExecStats on www.qdpma.com

  3. Overview • Why performance is still important today? • Brute force? • Yes, but … • Special Topics • Automating data collections • SQL Server Engine • What developers/DBA need to know?

  4. CPU & Memory 2001 and 2012 P P L2 PCI-E PCI-E PCI-E PCI-E PCI-E PCI-E PCI-E PCI-E PCI-E PCI-E FSB PCI-E PCI-E MCH QPI QPI QPI QPI PCI-E PCI-E PCI-E PCI-E P P C3 C3 C3 C3 LLC LLC LLC LLC C4 C4 C4 C4 QPI C2 C2 C2 C2 C5 C5 C5 C5 2001 – 4 sockets, 4 cores Pentium III Xeon, 900MHz 4-8GB memory? Xeon MP 2002-4 C1 C1 C1 C1 C6 C6 C6 C6 C0 C0 C0 C0 C7 C7 C7 C7 MI MI MI MI MI MI MI MI QPI QPI DMI 2 PCI-E PCI-E PCI-E PCI-E PCI-E QPI 2012 – 4 sockets, 8 cores each 4 x 8 = 32 cores total 768GB (48 x 16GB), Westmere-EX 1TB • 15 cores in next generation? Each core today is more than 10x over PIII PCI-E PCI-E PCI-E PCI-E PCI-E PCI-E 16GB $191 32GB $794

  5. Storage 2001 versus 2012/13 QPI 192 GB 192 GB SSD SSD SSD SSD QPI PCIe x4 PCIe x8 PCIe x8 PCIe x8 PCIe x8 PCIe x8 HDD HDD HDD HDD 10GbE IB RAID RAID RAID RAID 2001 100 x 10K HDD 125 IOPS each = 12.5K IOPS IO Bandwidth limited: 1.3GB/s (1/3 memory bandwidth) 2013 64 SSDs, >10K+ IOPS each, 1M IOPS possible IO Bandwidth 10GB/s easy SAN vendors – questionable BW

  6. Performance Past, Present, Future • When will servers be so powerful that … • Been saying this for a long time • Today – 10 to 100X overkill • 32-cores, 60-cores later in 2013? • Enough memory that IO is only sporadic • Unlimited IOPS with SSD • What can go wrong? Today’s topic

  7. Special Topics • Data type mistmatch • Multiple Optional Search Arguments (SARG) • Function on SARG • Parameter Sniffing versus Variables • Statistics related (big topic) • first OR, then AND/OR combinations • Complex Query with sub-expressions • Parallel Execution Not in order of priority

  8. 1a. Data type mismatch DECLARE@namenvarchar(25) = N'Customer#000002760' SELECT * FROM CUSTOMER WHERE C_NAME = @name SELECT* FROM CUSTOMER WHERE C_NAME = CONVERT(varchar, @name) .NET auto-parameter discovery? Unable to use index seek

  9. 1b. Type Mismatch – Row Estimate SELECT* FROM CUSTOMER WHERE C_NAME LIKEN'Customer#00000276%' SELECT* FROM CUSTOMER WHERE C_NAME LIKE'Customer#00000276%' Row estimate error could have severe consequences in a complex query

  10. SELECT TOP plus Row Estimate Error SELECT TOP 1000[Document].[ArtifactID]FROM[Document](NOLOCK)WHERE[Document].[AccessControlListID_D]IN(1,1000064,1000269)AND EXISTS (SELECT[DocumentBatch].[BatchArtifactID]FROM[DocumentBatch](NOLOCK)INNER JOIN[Batch](NOLOCK)ON[Batch].ArtifactID=[DocumentBatch].[BatchArtifactID]WHERE [DocumentBatch].[DocumentArtifactID]=[Document].[ArtifactID]AND[Batch].[Name]LIKEN'%Value%')ORDER BY[Document].[ArtifactID] Data type mismatch – results in estimate rows high Top clause – easy to find first 1000 rows In fact, there are few rows that match SARG Wrong plan for evaluating large number of rows http://www.qdpma.com/CBO/Relativity.html

  11. 2. Multiple Optional SARG DECLARE@Orderkeyint, @Partkeyint = 1 SELECT* FROMLINEITEM WHERE (@Orderkey IS NULL OR L_ORDERKEY = @Orderkey) AND (@PartkeyIS NULL OR L_PARTKEY = @Partkey) AND (@Partkey IS NOT NULL OR @Orderkey IS NOT NULL)

  12. Dynamically Built Parameterized SQL DECLARE@Orderkeyint,@Partkeyint=1 ,@SQLnvarchar(500),@Paramnvarchar(100) SELECT@SQL= N‘/* Comment */ SELECT * FROM LINEITEM WHERE 1=1‘ ,@Param=N'@Orderkey int, @Partkey int' IF (@OrderkeyISNOTNULL) SELECT@SQL=@SQL+N' AND L_ORDERKEY = @Orderkey' IF (@PartkeyISNOTNULL) SELECT@SQL=@SQL+N' AND L_PARTKEY = @Partkey' PRINT@SQL execsp_executesql@SQL,@Param,@Orderkey,@Partkey IF block is easier for few options Dynamically built parameterized SQL better for many options Considering /*comment*/ to help identify this

  13. IF block DECLARE@Orderkeyint, @Partkeyint = 1 IF (@OrderkeyISNOTNULL) SELECT* FROMLINEITEM WHERE (L_ORDERKEY = @Orderkey) AND(@PartkeyIS NULL OR L_PARTKEY = @Partkey) ELSE IF (@PartkeyISNOTNULL) SELECT * FROM LINEITEM WHERE(L_PARTKEY = @Partkey)

  14. 2b. Function on column SARG SELECTCOUNT(*), SUM(L_EXTENDEDPRICE) FROM LINEITEM WHEREYEAR(L_SHIPDATE) = 1995 ANDMONTH(L_SHIPDATE) = 1 SELECTCOUNT(*), SUM(L_EXTENDEDPRICE) FROM LINEITEM WHEREL_SHIPDATE BETWEEN'1995-01-01'AND'1995-01-31' DECLARE@Startdate date, @Daysint = 1 SELECTCOUNT(*), SUM(L_EXTENDEDPRICE) FROMLINEITEM WHEREL_SHIPDATE BETWEEN@Startdate ANDDATEADD(dd,1,@Startdate)

  15. Estimated versus Actual Plan - rows Estimated Plan – 1 row??? Actual Plan – actual rows 77,356

  16. 3 Parameter Sniffing -- first call, procedure compiles with these parameters execp_Report @startdate = '2011-01-01', @enddate = '2011-12-31' -- subsequent calls, procedure executes with original plan execp_Report @startdate = '2012-01-01', @enddate = '2012-01-07' Need different execution plans for narrow and wide range Options: 1) WITH RECOMPILE 2) main procedure calls 1 of 2 identical sub-procedures One sub-procedure is only called for narrow range Other called for wide range

  17. 4 Statistics • Auto-recompute points • Sampling strategy • Percentage • Random pages versus random rows • Histogram Equal and Range Rows • Out of bounds, value does not exist • etc Statistics Used by the Query Optimizer in SQL Server 2008 Writer: Eric N. Hanson and Yavor Angelov Contributor: Lubor Kollar http://msdn.microsoft.com/en-us/library/dd535534.aspx

  18. Statistics Structure • Stored (mostly) in binary field Scalar values Density Vector Histogram

  19. Statistics Auto/Re-Compute • Automatically generated on query compile • Recompute at 6 rows, 500, every 20%? Has this changed?

  20. Statistics Sampling • Sampling theory • True random sample • Sample error - square root N • Relative error 1/ N • SQL Server sampling • All rows in random pages

  21. Row Estimate - Statistics • Skewed data distribution • Out of bounds • Value does not exist

  22. Loop Join - Table Scan on Inner Source Estimated out from first 2 tabes (at right) is zero or 1 rows. Most efficient join to third table (without index on join column) is a loop join with scan. If row count is 2 or more, then a fullscan is performed for each row from outer source Default statistics rules may lead to serious ETL issues Consider custom strategy

  23. Compile Parameter Not Exists Main procedure has cursor around view_Servers First server in view_Servers is ’CAESIUM’ Cursor executes sub-procedure for each Server sql: SELECTMAX(ID) FROM TReplWS WHERE Hostname = @ServerName But CAESIUM does not exist in TReplWS!

  24. Good and Bad Plan?

  25. SqlPlan Compile Parameters

  26. SqlPlan Compile Parameters <?xmlversion="1.0"encoding="utf-8"?> <ShowPlanXMLxmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan"Version="1.1"Build="10.50.2500.0"> <BatchSequence> <Batch> <Statements> <StmtSimpleStatementText="@ServerName varchar(50) SELECT @maxid = ISNULL(MAX(id),0) FROM TReplWS WHERE Hostname = @ServerName" StatementId="1"StatementCompId="43"StatementType="SELECT"StatementSubTreeCost="0.0032843"StatementEstRows="1" StatementOptmLevel="FULL"QueryHash="0x671D2B3E17E538F1"QueryPlanHash="0xEB64FB22C47E1CF2" StatementOptmEarlyAbortReason="GoodEnoughPlanFound"> <StatementSetOptionsQUOTED_IDENTIFIER="true"ARITHABORT="false"CONCAT_NULL_YIELDS_NULL="true"ANSI_NULLS="true" ANSI_PADDING="true"ANSI_WARNINGS="true"NUMERIC_ROUNDABORT="false" /> <QueryPlanCachedPlanSize="16"CompileTime="1"CompileCPU="1"CompileMemory="168"> <RelOpNodeId="0"PhysicalOp="Compute Scalar"LogicalOp="Compute Scalar" EstimateRows="1"EstimateIO="0"EstimateCPU="1e-007“ AvgRowSize="15"EstimatedTotalSubtreeCost="0.0032843"Parallel="0"EstimateRebinds="0"EstimateRewinds="0"> </RelOp> <ParameterList> <ColumnReferenceColumn="@ServerName"ParameterCompiledValue="'CAESIUM'" /> </ParameterList> </QueryPlan> </StmtSimple> </Statements> </Batch> </BatchSequence> </ShowPlanXML> Compile parameter values at bottom of sqlplan file

  27. 5a Single Table OR -- Single table SELECT*FROMLINEITEM WHEREL_ORDERKEY= 1 ORL_PARTKEY=184826

  28. 5a Join 2 Tables, OR in SARG -- subsequent calls, procedure executes with original plan SELECTO_ORDERDATE,O_ORDERKEY,L_SHIPDATE,L_QUANTITY FROMLINEITEM INNERJOINORDERSONO_ORDERKEY=L_ORDERKEY WHEREL_PARTKEY=184826 ORO_CUSTKEY= 137099

  29. 5a UNION instead of OR SELECTO_ORDERDATE,O_ORDERKEY,L_SHIPDATE,L_QUANTITY,O_CUSTKEY,L_PARTKEY FROMLINEITEMINNERJOINORDERSONO_ORDERKEY=L_ORDERKEY WHEREL_PARTKEY= 184826 UNION SELECTO_ORDERDATE,O_ORDERKEY,L_SHIPDATE,L_QUANTITY,O_CUSTKEY,L_PARTKEY FROMLINEITEMINNERJOINORDERSONO_ORDERKEY=L_ORDERKEY WHEREO_CUSTKEY= 137099 Caution, select list should have keys to ensure correct rows UNION removes duplicates

  30. 5b AND/OR Combinations SELECT xx FROM A WHERE col1 IN (expr1) AND col2 NOT IN (expr2) SELECT xx FROM A WHERE (expr1) AND (expr2 OR expr3) • Hash Join is good method to process many rows • Requirement is equality join condition • In complex SQL with AND/OR or IN NOT IN combinations • Query optimizer may not be to determine that equality join condition exists • Execution plan will use loop join, • and attempt to force hash join will be rejected • Re-write using UNION in place of OR • And LEFT JOIN in place of NOT IN More on AND/OR combinations: http://www.qdpma.com/CBO/Relativity3.html

  31. Complex Query with Sub-expression • Query complexity – really high compile cost • Repeating sub-expressions (including CTE) • Must be evaluated multiple times • Main Problem • Row estimate error propagation • Solution • Temp table when estimate is high, actual is low More on AND/OR combinations: http://www.qdpma.com/CBO/Relativity4.html

  32. Parallelism • Designed for 1998 era • Cost Threshold for Parallelism: default 5 • Max Degree of Parallelism – instance level • OPTION (MAXDOP n) – query level • Today – complex system – 32 cores • Plan cost 5 query might run in 10ms? • Some queries at DOP 4 • Others at DOP 16? More on Parallelism: http://www.qdpma.com/CBO/ParallelismComments.html http://www.qdpma.com/CBO/ParallelismOnset.html

  33. Full-Text Search Loop Join with FT as inner Source Full Text search Potentially executed many times

  34. Summary • Hardware today is really powerful • Storage may not be – SAN vendor disconnect • Look for serious blunders first

  35. Special Topics • Data type mistmatch • Multiple Optional Search Arguments (SARG) • Function on SARG • Parameter Sniffing versus Variables • Statistics related (big topic) • AND/OR • Complex Query with sub-expressions • Parallel Execution

  36. Parallelism

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