module 12: optimizing query performance. overview introducing the query optimizer tuning performance...

Module 12:Optimizing Query

Performance

Overview

Introducing the Query Optimizer

Tuning Performance Using SQL Utilities

Using an Index to Cover a Query

Overriding the Query Optimizer

Understanding Indexing Strategies and Guidelines

Lesson: Introducing the Query Optimizer

Function of the Query Optimizer

Cost-Based Optimization

The Query Optimization Process

Phases of Query Optimization

Execution Plan Cache Management

The Query Governor

Function of the Query Optimizer

Determines the Most Efficient Execution Plan

Determines whether indexes exist and evaluatingtheir usefulness

Determines which indexes or columns can be used Determines how to process joins Uses cost-based evaluation of alternatives Creates column statistics

Uses Additional Information

Produces an Execution Plan

Cost-Based Optimization

Limits the Number of Optimization Plans

Cost is estimated in terms of I/O and CPU cost

Determines Query Processing Time

Use of physical operators and sequence of operations Use of parallel and serial processing

Results Set

Results Set

The Query Optimization Process

Standardization ProcessStandardization Process

CompilationCompilation

Data Access RoutinesData Access Routines

Query OptimizationQuery Optimization

Transact-SQLTransact-SQL Parsing ProcessParsing Process

Phases of Query Optimization

Query Analysis

Identifies the search and join criteria of the query

Index Selection

Determines whether an index or indexes exist Assesses the usefulness of the index or indexes

Join Selection

Evaluates which join strategy to use

Execution Plan Cache Management

Memory Storage

One copy for all serial executions Another copy for all parallel executions

Execution Context

An existing execution plan is reused, if one exists A new execution plan is generated, if one does not exist

Recompilation

Changes in database cause execution plan to be inefficient or invalid

The Query Governor

How Do You Prevent Long-Running Queries?

Use the query governor to specify an upper limit

Connection Limits

Use the sp_configure stored procedure

Execute the SET QUERY_GOVERNOR_COST_LIMIT statement

Specify 0 to turn off the query governor

sp_configure query governor cost limit valuesp_configure query governor cost limit value

SET QUERY_GOVERNOR_COST_LIMIT valueSET QUERY_GOVERNOR_COST_LIMIT value

Lesson: Tuning Performance Using SQL Utilities

The STATISTICS Statement

The SHOWPLAN_ALL and SHOWPLAN_TEXT Statements

The Graphical Execution Plan

Analysis of Graphical Execution Plan Output

Potential Problem Areas in Execution Plans

Statement Output Sample

STATISTICS TIME

SQL Server Execution Times:CPU time = 0 ms, elapsed time = 2 ms.

STATISTICS PROFILE

Rows Executes StmtText StmtId…

-----------------------------------------------

47 1 SELECT * FROM [charge] 16

WHERE (([charge_amt]>=@1)

.

.

.

STATISTICS IOTable 'member'. Scan count 1,

logical reads 23, physical reads 0, read-ahead reads 0.

The STATISTICS Statement

The SHOWPLAN_ALL and SHOWPLAN_TEXT Statements

Structure of the SHOWPLAN Statement Output

Returns information as a set of rows Forms a hierarchical tree Represents steps taken Shows estimated values of how a query was optimized

Details of the Execution Steps

SHOWPLAN_ALL Returns Additional Information

The Graphical Execution Plan

Steps Are Units of Work to Process a Query

Sequence of Steps Is the Order in Which the StepsAre Processed

Logical Operators Describe Relational Algebraic Operation Used to Process a Statement

Physical Operators Describe Physical Implementation Algorithm Used to Process a Statement

Analysis of Graphical Execution Plan Output

Query Plan

SELECT Cost: 0%

Bookmark Lookup Cost: 8%

Hash Match Root… Cost 28%

Member.corp_no Cost 9%

Member.fname Cost: 10%

Filter Cost: 0%

Sequence of Steps

Potential Problem Areas in Execution Plans

Problem Areas Solution

Table ScansAdd indexes to columns in theWHERE clause

Ensure the WHERE clause is selective

Bookmark Lookup Operator

Adding or modifying a nonclustered index to cover all columns

Hash Join Operations Adding an index to the Probe Input

Sort Operations Adding an index on the column in an ORDER BY or GROUP BY clause

Lesson: Using an Index to Cover a Query

Introduction to Indexes That Cover a Query

Single-Page Navigation

Partial-Scan Navigation

Full-Scan Navigation

Factors Affecting an Index’s Ability to Cover a Query

Methods to Verify That an Index Is Used to Cover a Query

Guidelines for Creating Indexes That Cover a Query

Introduction to Indexes That Cover a Query

Only Nonclustered Indexes Cover Queries

Indexes Must Contain All Columns Referenced in the Query

No Data Page Access Is Required

Indexed Views Can Pre-Aggregate Data

Indexes That Cover Queries Retrieve Data Quickly

Index Pages

Leaf Level (Key Value)

Non-Leaf LevelNon-Leaf Level

Single-Page Navigation

AkersAkersBarrBarr

BarretoBarretoBorbaBorbaBudaiBudai

KimKim……………………

LangLangMartinMartinMartinMartinMartinMartinMorrisMorris

EricEric……………………

…

Data Pages

… …

AkersAkersLangLang

…………

KimKimEricEric…………

AkersAkers……

GanioGanio……

KimKim……

JonJon……

LangLang………………

EricEric………………

SELECT lastname, firstnameFROM memberWHERE lastname = 'Hall'

GanioGanioHallHallHartonoHartonoJonesJonesJonesJones

JonJonDonDonTommyTommyBrannonBrannon

DavidDavid

Hall Don

Index Pages



Partial-Scan Navigation

AkersAkersBarrBarr


…………………………

GanioGanioHallHall

HartonoHartonoJonesJonesJonesJones

…………………………

MoyerMoyerNashNashNayNay

OwenOwenRussoRusso

…………………………

ChaiChaiConConCoxCoxCoxCox

DannerDanner

…………………………

DuboisDuboisDuffyDuffyFineFineFortFortFunkFunk

…………………………

JordanJordanKimKimKimKimKochKochKochKoch

…………………………


…………………………

SmithSmithSmithSmithSmithSmithSmithSmithSmithSmith

…………………………

AkersAkersChaiChai

DuboisDuboisGanioGanio

……………………

JordanJordanLangLang

MoyerMoyerSmithSmith

……………………

Data Pages

AkersAkers

JordanJordan

……

……

……

……

USE creditSELECT lastname, firstname FROM memberWHERE lastname BETWEEN 'Funk' AND 'Lang'

Index Pages



Data Pages

Full-Scan Navigation

AkersAkersBarrBarr


…………………………

GanioGanioHallHall

HartonoHartonoJonesJonesJonesJones

…………………………

MoyerMoyerNashNashNayNay

OwenOwenRussoRusso

…………………………

MartinMartinSmithSmith

……

AkersAkersGanioGanio

……

AkersAkers……

MartinMartin

ChaiChaiConConCoxCoxCoxCox

DannerDanner

…………………………

DuboisDuboisDuffyDuffyFineFineFortFortFunkFunk

…………………………

JordanJordanKimKimKimKimKochKochKochKoch

…………………………


…………………………

SmithSmithSmithSmithSmithSmithSmithSmithSmithSmith

…………………………

…

USE creditSELECT lastname, firstnameFROM member

Factors Affecting an Index’s Ability to Cover a Query

All Necessary Data Must Be in the Index

A Composite Index Is Useful Even if the First Column Is Not Referenced

A WHERE Clause Is Not Necessary

A Nonclustered Index Can Be Used if It Requires Less I/O Than a Clustered Index Containing a Column Referenced in the WHERE Clause

Indexes Can Be Joined to Cover a Query

Methods to Verify That an Index Is Used to Cover a Query

Execution Plan Output

Displays the phrase “Scanning a non-clustered index entirely or only a range”

No Bookmark Lookup operator

I/O Comparison

Nonclustered indexTotal number of levels in the non–leaf level

Total number of pages that make up the leaf level

Total number of rows per leaf-level page

Total number of rows per data page

Total number of pages that make up the table

Guidelines for Creating Indexes That Cover a Query

Add Columns to Indexes

Minimize Index Key Size

Maintain Row-to-Key Size Ratio

Lesson: Overriding the Query Optimizer

Hints and SET FORCEPLAN Statement

Guidelines for Maintaining Overridden Queries

Alternatives to Overriding the Query Optimizer

Hints and SET FORCEPLAN Statement

Table Hints

Join Hints

Query Hints

SET FORCEPLAN Statement

Guidelines for Maintaining Overridden Queries

Verify That Performance Improves

Document Reasons for Using Optimizer Hints

Retest Queries Regularly

Alternatives to Overriding the Query Optimizer

Limit Optimizer Hints

Explore Other Alternatives Before Overriding the Query Optimizer by:

Updating statistics Recompiling stored procedures Reviewing the queries or search arguments Evaluating the possibility of building different indexes

Lesson: Understanding Indexing Strategies and Guidelines

I/O Consideration and Evaluation

Indexing Strategy for Multiple Queries

Guidelines for Creating Indexes

Best Practices

I/O Consideration and Evaluation

SELECT charge_noFROM chargeWHERE charge_amt BETWEEN 20 AND 30

SELECT charge_noFROM chargeWHERE charge_amt BETWEEN 20 AND 30

Access method Approx Page I/O

Table scan 10,417

Clustered index on the charge_amt column 1,042

Nonclustered index on the charge_amt column 100,273

Composite index on charge_amt, charge_no columns 273

Example 2Example 2

Indexing Strategy for Multiple Queries

USE creditSELECT charge_no, charge_dt, charge_amtFROM chargeWHERE statement_no = 19000 AND member_no = 3852

USE creditSELECT charge_no, charge_dt, charge_amtFROM chargeWHERE statement_no = 19000 AND member_no = 3852

USE creditSELECT member_no, charge_no, charge_amtFROM chargeWHERE charge_dt BETWEEN '07/30/1999'AND '07/31/1999' AND member_no = 9331

USE creditSELECT member_no, charge_no, charge_amtFROM chargeWHERE charge_dt BETWEEN '07/30/1999'AND '07/31/1999' AND member_no = 9331

Example 1Example 1

Guidelines for Creating Indexes

Determine the Priorities of All of the Queries

Determine the Selectivity for Each Portion of the WHERE Clause of Each Query

Determine Whether to Create an Index

Identify the Columns That Should Be Indexed

Determine the Best Column Order of Composite Indexes

Determine What Other Indexes Are Necessary

Test the Performance of the Queries

Use the Query Governor to Prevent Long-Running Queries from Consuming System Resources Use the Query Governor to Prevent Long-Running Queries from Consuming System Resources Have a Thorough Understanding of the Data and How Queries Gain Access to Data Have a Thorough Understanding of the Data and How Queries Gain Access to Data Create Indexes That Cover the Most Frequently Used QueriesCreate Indexes That Cover the Most Frequently Used QueriesEstablish Indexing Strategies for Individual and Multiple QueriesEstablish Indexing Strategies for Individual and Multiple Queries

Avoid Overriding the Query Optimizer Avoid Overriding the Query Optimizer

Best Practices

Lab A: Optimizing Query Performance

Exercise 1: Using the Graphical Execution Plan to Determine How a Query Is Resolved

Exercise 2: Comparing I/O for Queries That Are Covered or Not Coveredby Indexes

Exercise 3: Comparing I/O for Queries That Retrieve a Range of Data

If Time Permits:Using Optimizer Hints to Force the Use of an Index or Join

Course Evaluation

module 12: optimizing query performance. overview introducing the query optimizer tuning performance...

Documents

query optimizerdetermines

query optimizerfunction

query governorsp

query sequence of steps

invalidthe query governorhow

execution steps showplan

execution planslesson

fname cost