#SQLSatRiyadh

Special TopicsJoe 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

– writing your own statistics• Disk IO cost structure• Tools for system monitoring, execution plan analysis

See ExecStats on www.qdpma.com

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?

CPU & Memory 2001 and 2012

2001 – 4 sockets, 4 coresPentium III Xeon, 900MHz 4-8GB memory?

Xeon MP 2002-4

2012 – 4 sockets, 8 cores each4 x 8 = 32 cores total768GB (48 x 16GB), Westmere-EX 1TB15 cores in next generation?

FSB

PL2

P P P

MCH

QPI

QPI

DM

I 2

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

QPI

QPI

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

MI

PCI-E

C1 C6C2 C5C3 C4

LLC

QPI

MIC7C0

MI

PCI-E

C1 C6C2 C5C3 C4

LLC

QPI

MIC7C0

MI

PCI-E

C1 C6C2 C5C3 C4

LLC

QPI

MIC7C0

MI

PCI-E

C1 C6C2 C5C3 C4

LLC

QPI

MIC7C0

Each core today is more than 10x over PIII

16GB $19132GB $794

Storage 2001 versus 2012/13

PC

Ie x8

PC

Ie x8

PC

Ie x8

PC

Ie x8

PC

Ie x8

PC

Ie x4

IBRAID RAID RAIDRAID10GbE

QPI

QPI192 GB 192 GB

HDD HDD HDD HDD

SSD SSD SSD SSD

2001 100 x 10K HDD 125 IOPS each = 12.5K IOPSIO Bandwidth limited: 1.3GB/s (1/3 memory bandwidth)

201364 SSDs, >10K+ IOPS each, 1M IOPS possibleIO Bandwidth 10GB/s easy

SAN vendors – questionable BW

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

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

1a. Data type mismatchDECLARE @name nvarchar(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

1b. Type Mismatch – Row EstimateSELECT * FROM CUSTOMER WHERE C_NAME LIKE N'Customer#00000276%'SELECT * FROM CUSTOMER WHERE C_NAME LIKE 'Customer#00000276%'

Row estimate error could have severe consequences in a complex query

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] LIKE N'%Value%' ) ORDER BY [Document].[ArtifactID]

Data type mismatch – results in estimate rows highTop clause – easy to find first 1000 rows

In fact, there are few rows that match SARGWrong plan for evaluating large number of rows

http://www.qdpma.com/CBO/Relativity.html

2. Multiple Optional SARGDECLARE @Orderkey int, @Partkey int = 1

SELECT * FROM LINEITEM WHERE (@Orderkey IS NULL OR L_ORDERKEY = @Orderkey) AND (@Partkey IS NULL OR L_PARTKEY = @Partkey)

AND (@Partkey IS NOT NULL OR @Orderkey IS NOT NULL)

Dynamically Built Parameterized SQLDECLARE @Orderkey int, @Partkey int = 1, @SQL nvarchar(500), @Param nvarchar(100)SELECT @SQL = N‘/* Comment */SELECT * FROM LINEITEM WHERE 1=1‘, @Param = N'@Orderkey int, @Partkey int'

IF (@Orderkey IS NOT NULL) SELECT @SQL = @SQL + N' AND L_ORDERKEY = @Orderkey'IF (@Partkey IS NOT NULL) SELECT @SQL = @SQL + N' AND L_PARTKEY = @Partkey'PRINT @SQLexec sp_executesql @SQL, @Param, @Orderkey, @Partkey

IF block is easier for few optionsDynamically built parameterized SQL better for many optionsConsidering /*comment*/ to help identify this

IF blockDECLARE @Orderkey int, @Partkey int = 1

IF (@Orderkey IS NOT NULL) SELECT * FROM LINEITEM WHERE (L_ORDERKEY = @Orderkey) AND (@Partkey IS NULL OR L_PARTKEY = @Partkey)

ELSE IF (@Partkey IS NOT NULL) SELECT * FROM LINEITEM WHERE (L_PARTKEY = @Partkey)

2b. Function on column SARGSELECT COUNT(*), SUM(L_EXTENDEDPRICE) FROM LINEITEM WHERE YEAR(L_SHIPDATE) = 1995 AND MONTH(L_SHIPDATE) = 1

SELECT COUNT(*), SUM(L_EXTENDEDPRICE) FROM LINEITEM WHERE L_SHIPDATE BETWEEN '1995-01-01' AND '1995-01-31'

DECLARE @Startdate date, @Days int = 1SELECT COUNT(*), SUM(L_EXTENDEDPRICE) FROM LINEITEM WHERE L_SHIPDATE BETWEEN @Startdate AND DATEADD(dd,1,@Startdate)

Estimated versus Actual Plan - rowsEstimated Plan – 1 row???

Actual Plan – actual rows 77,356

3 Parameter Sniffing-- first call, procedure compiles with these parametersexec p_Report @startdate = '2011-01-01', @enddate = '2011-12-31'

-- subsequent calls, procedure executes with original planexec p_Report @startdate = '2012-01-01', @enddate = '2012-01-07'

Need different execution plans for narrow and wide rangeOptions: 1) WITH RECOMPILE2) main procedure calls 1 of 2 identical sub-proceduresOne sub-procedure is only called for narrow rangeOther called for wide range

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 2008Writer: Eric N. Hanson and Yavor AngelovContributor: Lubor Kollarhttp://msdn.microsoft.com/en-us/library/dd535534.aspx

Statistics Structure

• Stored (mostly) in binary field Scalar values

Density Vector

Histogram

Statistics Auto/Re-Compute

• Automatically generated on query compile• Recompute at 6 rows, 500, every 20%?

Has this changed?

Statistics Sampling

• Sampling theory– True random sample– Sample error - square root N • Relative error 1/ N

• SQL Server sampling– All rows in random pages

Row Estimate - Statistics

• Skewed data distribution• Out of bounds• Value does not exist

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 issuesConsider custom strategy

Compile Parameter Not ExistsMain procedure has cursor around view_ServersFirst server in view_Servers is ’CAESIUM’Cursor executes sub-procedure for each Serversql:

SELECT MAX(ID) FROM TReplWS WHERE Hostname = @ServerName

But CAESIUM does not exist in TReplWS!

Good and Bad Plan?

SqlPlan Compile Parameters

SqlPlan Compile Parameters<?xml version="1.0" encoding="utf-8"?><ShowPlanXML xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan" Version="1.1" Build="10.50.2500.0"> <BatchSequence> <Batch> <Statements> <StmtSimple StatementText="@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">

<StatementSetOptions QUOTED_IDENTIFIER="true" ARITHABORT="false" CONCAT_NULL_YIELDS_NULL="true" ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" NUMERIC_ROUNDABORT="false" />

<QueryPlan CachedPlanSize="16" CompileTime="1" CompileCPU="1" CompileMemory="168"> <RelOp NodeId="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> <ColumnReference Column="@ServerName" ParameterCompiledValue="'CAESIUM'" /> </ParameterList> </QueryPlan> </StmtSimple> </Statements> </Batch> </BatchSequence></ShowPlanXML>

Compile parameter values at bottom of sqlplan file

5a Single Table OR-- Single tableSELECT * FROM LINEITEM WHERE L_ORDERKEY = 1 OR L_PARTKEY = 184826

5a Join 2 Tables, OR in SARG-- subsequent calls, procedure executes with original planSELECT O_ORDERDATE, O_ORDERKEY, L_SHIPDATE, L_QUANTITYFROM LINEITEM INNER JOIN ORDERS ON O_ORDERKEY = L_ORDERKEYWHERE L_PARTKEY = 184826 OR O_CUSTKEY = 137099

5a UNION instead of ORSELECT O_ORDERDATE, O_ORDERKEY, L_SHIPDATE, L_QUANTITY, O_CUSTKEY, L_PARTKEY FROM LINEITEM INNER JOIN ORDERS ON O_ORDERKEY = L_ORDERKEY WHERE L_PARTKEY = 184826UNIONSELECT O_ORDERDATE, O_ORDERKEY, L_SHIPDATE, L_QUANTITY, O_CUSTKEY, L_PARTKEY FROM LINEITEM INNER JOIN ORDERS ON O_ORDERKEY = L_ORDERKEY WHERE O_CUSTKEY = 137099

Caution, select list should have keys to ensure correct rowsUNION removes duplicates

5b AND/OR Combinations• 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

SELECT xx FROM A WHERE col1 IN (expr1) AND col2 NOT IN (expr2)SELECT xx FROM A WHERE (expr1) AND (expr2 OR expr3)

More on AND/OR combinations: http://www.qdpma.com/CBO/Relativity3.html

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

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.htmlhttp://www.qdpma.com/CBO/ParallelismOnset.html

Full-Text Search

Loop Join with FT as inner Source Full Text search Potentially executed many times

Summary

• Hardware today is really powerful– Storage may not be – SAN vendor disconnect

• Look for serious blunders first

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

Parallelism