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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

Special Topics

Joe Chang

www.qdpma.com

about joe
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
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
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

storage 2001 versus 2012 13
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

performance past present future
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 topics1
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 mismatch
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

1b type mismatch row estimate
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

select top plus row estimate error
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

2 multiple optional sarg
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)

dynamically built parameterized sql
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

if block
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)

2b function on column sarg
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)

estimated versus actual plan rows
Estimated versus Actual Plan - rows

Estimated Plan – 1 row???

Actual Plan – actual rows 77,356

3 parameter sniffing
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

4 statistics
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

statistics structure
Statistics Structure
  • Stored (mostly) in binary field

Scalar values

Density Vector

Histogram

statistics auto re compute
Statistics Auto/Re-Compute
  • Automatically generated on query compile
  • Recompute at 6 rows, 500, every 20%?

Has this changed?

statistics sampling
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
Row Estimate - Statistics
  • Skewed data distribution
  • Out of bounds
  • Value does not exist
loop join table scan on inner source
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

compile parameter not exists
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!

sqlplan compile parameters1
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

5a single table or
5a Single Table OR

-- Single table

SELECT*FROMLINEITEM

WHEREL_ORDERKEY= 1

ORL_PARTKEY=184826

5a join 2 tables or in sarg
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

5a union instead of or
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

5b and or combinations
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

complex query with sub expression
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
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

full text search
Full-Text Search

Loop Join with FT as inner

Source Full Text search

Potentially executed

many times

summary
Summary
  • Hardware today is really powerful
    • Storage may not be – SAN vendor disconnect
  • Look for serious blunders first
special topics2
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