Modern Performance - SQL Server

Joe Changwww.qdpma.com Jchang6 @ yahoo

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 http://www.qdpma.com/Download: http://www.qdpma.com/ExecStatsZip.htmlBlog: http://sqlblog.com/blogs/joe_chang/default.aspx

Overview

• General SQL Server Performance• Why performance is still important today?– Brute force?• Yes, but …

• Special Topics – spectacular fails• Automating data collections• SQL Server Engine– What developers/DBA need to know?

Not in this session

• List of rules to be followed blindly• without consideration for the underlying reason

• and whether rule actually applies in the current circumstance

DBA skill: cause and effect analysis & assessment

Common Themes?

• execution plan– Very large (multiple order of magnitude) error in

row estimate• Single (execute) of large operation – Might still be tolerable

• Multiple (executes) of large operations

select a.Header, a.CUSIP, a.SecNo, a.Security, a.Symbol ,a.Split_rep, a.Sales_Person_Name,cast(sum(a.January) as float) as January ,cast(sum(a.February) as float) as February ,cast(sum(a.March) as float) as March ,cast(sum(a.April) as float) as April ,cast(sum(a.May) as float) as May ,cast(sum(a.June) as float) as June ,cast(sum(a.July) as float) as July ,cast(sum(a.August) as float) as August ,cast(sum(a.September) as float) as September ,cast(sum(a.October) as float) as October ,cast(sum(a.November) as float) as November ,cast(sum(a.December) as float) as December ,cast(sum(a.Total) as float) as Totalfrom( select cast(hdr.Header as varchar(100)) as Header ,cast(AcctSec.CUSIP as varchar(100)) as CUSIP ,cast(AcctSec.Sec_No as varchar(100)) as SecNo ,cast(AcctSec.Sec_Desc1 as varchar(100)) as Security ,cast(AcctSec.Symbol as varchar(100)) as Symbol ,case when RefMonth.[MonthName] = 'January' then fct.Comm else 0 end as January ,case when RefMonth.[MonthName] = 'February' then fct.Comm else 0 end as February ,case when RefMonth.[MonthName] = 'March' then fct.Comm else 0 end as March ,case when RefMonth.[MonthName] = 'April' then fct.Comm else 0 end as April ,case when RefMonth.[MonthName] = 'May' then fct.Comm else 0 end as May ,case when RefMonth.[MonthName] = 'June' then fct.Comm else 0 end as June ,case when RefMonth.[MonthName] = 'July' then fct.Comm else 0 end as July ,case when RefMonth.[MonthName] = 'August' then fct.Comm else 0 end as August ,case when RefMonth.[MonthName] = 'September' then fct.Comm else 0 end as September ,case when RefMonth.[MonthName] = 'October' then fct.Comm else 0 end as October ,case when RefMonth.[MonthName] = 'November' then fct.Comm else 0 end as November ,case when RefMonth.[MonthName] = 'December' then fct.Comm else 0 end as December ,fct.Comm as Total ,AcctEmp.split_rep ,AcctEmp.Sales_Person_Name from PayoutSystemDW.[dbo].[PS_FactAccountSummary] fct join PayoutSystemDW.dbo.PS_DimensionRptBus RptBus on fct.DimRptBusID = RptBus.DimRptBusID join PayoutSystemDW.dbo.PS_DimensionHeader hdr on fct.DimHeaderID = hdr.DimHeaderID join PayoutSystemDW.dbo.PS_DimensionCurrency cur on fct.DimCurID = cur.DimCurID and cur.DimCurID = 1 join PayoutSystemDW.dbo.PS_DimensionAcctEmp AcctEmp on fct.DimAcctEmpID = acctemp.DimAcctEmpID and AcctEmp.Empno = 8125 and AcctEmp.Split_rep in ('PB54') join PayoutSystemDW.dbo.PS_DimensionAcctSec AcctSec on fct.DimAcctSecID = AcctSec.DimAcctSecID join PayoutSystemDW.dbo.PS_DimensionRefBuySell bs on fct.DimRefBuySellID = bs.DimRefBuySellID join PayoutSystemDW.[dbo].[PS_DimensionAcctOrg] AcctOrg on fct.DimAcctOrgID = AcctOrg.DimAcctOrgID and AcctOrg.OrgCode in ('38C') join PayoutSystemDW.[dbo].[PS_DimensionAcctClt] as AcctClt on AcctClt.DimAcctCltID = AcctClt.DimAcctCltID and AcctClt.ClientName = 'BRACY DENNIS M' join PayoutSystemDW.dbo.PS_DimensionTradeInd ti on ti.DimTradeIndID = fct.DimTradeIndID and ti.[Trade_Ind_Year] = 2014 join PayoutSystemDW.dbo.PS_DimensionRefMonth RefMonth on RefMonth.MonthID = ti.Trade_Ind_Month where RptBus.ReportID = 1) agroup by a.Header, a.CUSIP, a.SecNo, a.Security, a.Symbol,a.Split_rep,a.Sales_Person_Name

select fct.Comm as Total, …From FactAccountSummary fctjoin DimensionRptBus RptBus on fct.DimRptBusID = RptBus.DimRptBusIDJoin DimensionCurrency cur on fct.DimCurID = cur.DimCurIDjoin DimensionRefBuySell bs on fct.DimRefBuySellID = bs.DimRefBuySellIDjoin DimensionAcctOrg] AcctOrg on fct.DimAcctOrgID = AcctOrg.DimAcctOrgIDjoin DimensionAcctClt as AcctClt on AcctClt.DimAcctCltID = AcctClt.DimAcctCltID

QPI

CPU & Memory 2001 versus 2014

Xeon E7 v2 (Ivy Bridge), 15 cores, 3 QPI4 x 15 = 60 cores3TB (96 x 32GB) 24 DIMMs per socket40 PCI-E gen3 lanes + x4 g2 / socket

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

Xeon MP 2002-4

FSB

PL2

P P P

MCH

Each core today is more than 10x over Pentium III (700MHz?)

Mem___2013 __ 2014 16GB __ $191 __ $18032GB __ $794 __ $65064GB _____ __ $4510

PCH

DM

I

x4x4 x4 x4

MC

GFX

QPI

QPI

QPI

QPI

QPI

MI

PCI-E

MI

C1C2C3

C0

C4

C8C7C6

C9

C5

LLC111213

10

14QPI

MI

PCI-E

MI

C1C2C3

C0

C4

C8C7C6

C9

C5

LLC111213

10

14

QPI

MI

PCI-E

MI

C1C2C3

C0

C4

C8C7C6

C9

C5

LLC111213

10

14QPI

MI

PCI-E

MI

C1C2C3

C0

C4

C8C7C6

C9

C5

LLC111213

10

14

DM

I 2

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

DM

I 2

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

CPU & Memory 2001 versus 2012

Xeon E5 (Sandy Bridge), 8 cores, 2 QPI4 x 8 = 32 cores totalWestmere-EX 1TB (64x16GB) (3 QPI)Sandy Bridge E5: 768GB (48 x 16GB) (2 QPI)

FSB

PL2

P P P

MCH

QPI

QPI

DM

I 2

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

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

Xeon MP 2002-4

Each core today is more than 10x over Pentium III (700MHz?)

Mem___2013 __ 2014 16GB __ $191 __ $18032GB __ $794 __ $65064GB _____ __ $4510

QPI

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

PCI-EQPI

MI

PCI-E

C1 C6C2 C5C3 C4

LLC

QPI

MIC7C0

DM

I 2

PCI-E

PCI-E

PCI-E

PCI-E

PCI-E

MI

PCI-E

C1 C6C2 C5C3 C4

LLC

QPI

MIC7C0

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

Intel E5 & E7 v2 (Ivy-Bridge)

PCH

DM

I

x4x4 x4 x4

MC

GFX

E3 v3

Processor – Core

Microprocessor Pipeline

Branch PredictInstruction FetchDecodeRegister Allocate & RenameRe-Ordering BufferScheduleExecuteFlagsRetire

BP

BP IF ID RAT ROB Sch Exec Flags1st Retire

BP IF ID RAT ROB Sch Exec Flags2nd Retire

3GHz0.33ns clock

5 ns from start to finish200MHz

Microprocessor (core) is (multi-lane) assembly lineEach core is superscalarProcessor (socket) has multiple coresSystem has multiple sockets

Micro-architecture Sandy-Bridge

Haswell (Xeon E5/7 v3)

CPU Access Times

L1 I L1 DL2 Unified

L3 Slice

DRAM

Core – 3.33GHz 1 CPU cycle = 0.3ns

L1 cache – 4 CPU clocks (1ns)

L2 cache 12 CPU cycles (4ns?)

L3 cache 29+ cycles

Local node memory28 cycles + 49 ns (open page)28 cycles + 56 ns (random page)

Remote node (1-hop) memory 28 + 100ns

2-hop 150-300ns+?

Logical 0 Logical 1

Latency Orders of Magnitude

PCH

DM

I

x4x4 x4 x4

MC

GFX

CoreCore – 3.33GHz 1 CPU cycle = 0.3ns

L1 cache – 4 CPU clocks (1ns)

L2 cache 12 CPU cycles (4ns?)

L3 cache 29+ cycles

Local node memory28 cycles + 49 ns (open page)28 cycles + 56 ns (random page)

Remote node (1-hop) memory 28 + 100ns

2-hop 150-300ns+?

L1 Cache

L1 Cache

LLC

Westmere-EX 8-Socket System

QPI

QPI

QPI

QPI

QPI

IOH 0

QPI

IOH 1

QPI

IOH 2

QPI

QPI IOH 3

PCI-E x8PCI-E x8

PCI-E x4

ESI

PCI-E x8PCI-E x8 PCH

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

C2

C1C0

C7

C8C9

C4 C5QPI QPI

C3 C6

MC

LLC

MC

QPI QPI

Large server systems are very complicated

Software developed without consideration for system architecture will likely have severe problems

This applies to the OS, SQL Server and the application

SMB

SMB

SMB

SMB

SMB

SMB

SMB

SMB

Storage 2001 versus 2012/13QPI

QPI192 GB

PC

Ie x8

PC

Ie x8

PC

Ie x8

PC

Ie x8

PC

Ie x8

PC

Ie x4

IBRAID RAID RAIDRAID10GbE

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 total possible10-20GB/s+ IO Bandwidth easy6.4GB/s on each PCIe G3 x8

SAN vendors – questionable BW

PC

I

PC

I

PC

I

PC

I

MCH

RAID RAID RAID RAID

HDD

HDD

HDD

HDD

HDD

HDD

HDD

HDD

http://www.qdpma.com/Storage/Storage2013.htmlhttp://www.qdpma.com/ppt/Storage_2013.pptx

SAN

SSD 10K 7.2K Hot Spares

Auto-tier pools

Switch Switch

SP A SP B

8 Gb FC

x4 SAS 2GB/s

24 GB 24 GB

HBA HBA

PC

Ie

PC

Ie

or10Gb FCOE

0.8 GB/s

x4 SAS 2GB/s

Data 5 Data 6 Data 7

Data 1 Data 2 Data 3 Data 4

Data 8

Data 9

Data 13

Data 10

Data 14

Data 11

Data 15

Data 12

Data 16

SSD 1 SSD 2 SSD 3 SSD 4

Log 1 Log 2 Log 3 Log 4

Node 1

1024 GB

Node 2

1024 GB

Switch Switch

SP A SP B

8 Gb FC

24 GB 24 GB

SSD

x8 x8

SSD

x8 x8

x8x8

x8

SSD

x8

SSD

Node 1 Node 2

768 GB 768 GB

http://sqlblog.com/blogs/joe_chang/archive/2013/05/10/enterprise-storage-systems-emc-vmax.aspx http://sqlblog.com/blogs/joe_chang/archive/2013/02/25/emc-vnx2-and-vnx-future.aspx

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 in 2012, 60-cores in 2014– Enough memory that IO is only sporadic– Unlimited IOPS with SSD

• What can go wrong?

Today’s topic

SQL Performance

Natural keys with unique indexes, not SQL

The Execution Plan links all the elements of performanceIndex tuning alone has limited valueOver indexing can cause problems as well

Index and Statistics maintenance policy

1 Logic may need more than one execution plan?

Compile cost versus execution cost?

Tables and SQL combined implement business logic

Plan cache bloat?

SQL Tablesnatural keys

Indexes

Execution Plan

Statistics& Compile parameters

Compile Row estimate propagation

errors

Storage Engine

Hardware

DOP MemoryParallel plans

Recompiletemp table /

table variable

Query Optimizer

Index & Stats Maintenance

API Server Cursors: open, prepare, execute, close?

SET NO COUNT Information

messages

Factors to Consider

SQL Tables Indexes

Query Optimizer

Statistics

Compile Parameters

Storage Engine

Hardware

DOPmemory

Special Topics

• Data type mismatch• Multiple Optional Search Arguments (SARG)– Function on SARG

• Parameter Sniffing versus Variables• Statistics related (big topic)• OR, AND/OR combinations IN/NOT IN, EXISTS• Complex Query with sub-expressions• Parallel Execution

Not in order of priority

http://blogs.msdn.com/b/sqlcat/archive/2013/09/09/when-to-break-down-complex-queries.aspx

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

Table column is varcharParameter/variable is nvarchar

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

Row estimate error could have severe consequences in a complex query

SELECT TOP + 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

MULTIPLE OPTIONAL SARG

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)

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)

Need to consider impact of Parameter Sniffing,Consider the OPTIMIZER FOR hint

These are actually the stored procedure parameters

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 optionsConsider /*comment*/ to help identify source of SQL

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) OPTIMIZE FOR – one plan for all ranges2) WITH RECOMPILE – compile on each execute3) main procedure calls 1 of 2 identical sub-proceduresOne sub-procedure is only called for narrow rangeOther called for wide range

Skewed data distributions also importantExample: Large & small customers

Assuming date data type

STATISTICS

4 Statistics

• Auto-recompute points• Sampling strategy– How much to sample - theory?– 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 2008Eric N. Hanson and Yavor Angelov, Contributor: Lubor KollarOptimizing Your Query Plans with the SQL Server 2014 Cardinality EstimatorJoseph Sack

http://msdn.microsoft.com/en-us/library/dd535534.aspx

Statistics Structure

• Stored (mostly) in binary field Scalar values

Density Vector – limit 30, half in NC, half Cluster key

HistogramUp to 200 steps

Consider not blindly using IDENTITY on critical tablesExample: Large customers get low ID valuesSmall customers get high ID values

http://sqlblog.com/blogs/joe_chang/archive/2012/05/05/decoding-stats-stream.aspx

Statistics Auto/Re-Compute

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

Has this changed? 2008 R2Trace 2371 – lower threshold auto recomputed for large tables

http://support.microsoft.com/kb/2754171

Statistics Sampling

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

• SQL Server sampling– Random pages • But always first and last page???

– All rows in selected pages

Row Estimate Problems (at source)

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

Row estimate errors at source – is classified under statistics topic

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

AND – OR, IN / NOT IN, EXISTS / NOT EXISTS COMBINATIONS

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 (ALL) 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 = 184826UNION (ALL)SELECT 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 -- AND (L_PARTKEY <> 184826 OR L_PARTKEY IS NULL) --

Caution: select list should have keys to ensure correct rowsUNION removes duplicates (with Sort operation)UNION ALL does not

-- Hugo Kornelis trick --

5b AND/OR Combinations• Hash Join is good method to process many rows – Requirement is equality join condition–

• AND/OR, IN NOT IN, EXISTS NOT EXISTS 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 QUERIES

Complex Queries

• High Compile effort– Many joins, Many indexes– Estimated plan cost correlation

• Row estimation errors after multiple operations

Row estimate errors at source – is classified under statistics topic

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/Strategy – Get a good execution plan– Temp table when estimate is high, actual is low.

More on AND/OR combinations: http://www.qdpma.com/CBO/Relativity4.htmlhttp://blogs.msdn.com/b/sqlcat/archive/2013/09/09/when-to-break-down-complex-queries.aspx

When Estimate is low, and actual rows is high, need to balance temp table insert overhead versus plan benefit. Would a join hint work?

More Plan Details

Query with joining 6 tablesEach table has too many indexesRow estimate is high – plan cost is highQuery optimizer tries really really hard to find better planActual rows is moderate, any plan works

Temp Table and Table Variable

• Forget what other people have said– Most is cr@p

• Temp Tables – subject to statistics auto/re-compile

• Table variable – no statistics, assumes 1 row

• Question: In each specific case: does the statistics and recompile help or not?– Yes: temp table– No: table variable

Is this still true?

Row Estimate Error after Join

IO – synchronous when estimate rows is < 25, asynchronous when > 25

Row Estimate 2

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

Really need to rethink parallelism / NUMA strategies

Number of concurrently running queries x DOP less than number of logical/physical processors?

Tables with computed columns may inhibit parallelism?

Parallel Execution – or not?

Tables with computed columns using UDF prevent parallelism

Full-Text Search

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

varchar(max) stored in lob pages

• Disk IO to lob pages is synchronous?– Must access row to get 16 byte link?– Feature request: index pointer to lob

SQL PASS 2013Understanding Data Files at the Byte LevelMark Rasmussen

legacy

• API Server Cursors / Cursor Stored Procedures– sp_prepare / sp_prepexec, sp_execute,

sp_unprepare– sp_cursoropen, sp_cursorfetch, sp_cursorclose– sp_cursorprepare / sp_cursorprepexec,

sp_cursorexecute, sp_cursorunprepare

• Guess which is not called?– Symptom: sp_reset_connection

http://technet.microsoft.com/en-us/library/ms187088(v=sql.105).aspx API Server Cursorshttp://technet.microsoft.com/en-us/library/ms187801(v=sql.120).aspx Cursor Stored Procedures

Summary

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

• Standard performance practice– Top resource consumers, index usage

• But also Look for serious blunders

http://www.qdpma.com/CBO/SQLServerCostBasedOptimizer.htmlhttp://www.qdpma.com/CBO/Relativity.htmlhttp://blogs.msdn.com/b/sqlcat/archive/2013/09/09/when-to-break-down-complex-queries.aspxKevin Boles – Common TSQL Mistakes

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Special Topics

• Data type mismatch• Multiple Optional Search Arguments (SARG)– Function on SARG

• Parameter Sniffing versus Variables• Statistics related (big topic)• AND/OR• Complex Query• Parallel Execution

SQL Server Edition Strategies

• Enterprise Edition – per core licensing costs– Old system strategy• 4 (or 2)-socket server, top processor, max memory

– Today: How many cores are necessary• 2 socket system, max memory (16GB DIMMs)

• Is standard edition adequate– Low cost, but many important features disabled

• BI edition – 16 cores– Limited to 64GB for SQL Server process

New Features in SQL Server• 2005

– Index included columns– Filtered index– CLR

• 2008– Partitioning– Compression

• 2012– Column store (non-clustered)

• 2014– Column store clustered– Hekaton

GENERAL PERFORMANCEGeneral Performance

SQL Performance General

• Client-side architecture – Connection pooling– stored procedures versus SQL, parameterized

• Database Architecture– Cluster key, primary key, natural keys, foreign keys

• SQL – • Indexing• Indexes & Statistics Maintenance

Client-side Architecture

• Connection pooling: – Connection.Open, Execute, Connection.Close– Sp_reset_connection

• Stored procedures – parameterized SQL– Stored procedure name is short– Parameterized SQL may not be • Larger than 1 Ethernet packet? 2?, 8?

Database Architecture

• Normalization• Cluster key• Primary Key & other unique / natural keys• Foreign keys

Principles

TestingData

Server

NetworkStorage

QPI

CPU & Memory 2001 versus 2014x

Xeon E7 v2 (Ivy Bridge, 3 QPI)4 x 15 = 60 cores3TB (96 x 32GB) 24 DIMMs per socket40 PCI-E gen3 lanes + x4 g2 / socket

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

Xeon MP 2002-4

FSB

PL2

P P P

MCH

Each core today is more than 10x over Pentium III (700MHz?)

Mem___2013 __ 2014 16GB __ $191 __ $18032GB __ $794 __ $650

PCH

DM

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x4x4 x4 x4

MC

GFX

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Work in progress

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