5 great oracle myths

8/4/2019 5 Great Oracle Myths

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30/08/2011 Niall Litchfield - Audit Commission 1

Oracle MythsOracle Myths Avoiding Bad Ideas in OracleAvoiding Bad Ideas in Oracle

ManagementManagement

Niall LitchfieldNiall Litchfield

Oracle DBAOracle DBA


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Mything In ActionMything In Action

Managing Oracle is a complex task.Managing Oracle is a complex task.

Dont rely on folkloreDont rely on folklore

Dont believe everything you readDont believe everything you read Everyone screws up sometimesEveryone screws up sometimes


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Getting it rightGetting it right

Document your aimsDocument your aims

Document your assumptionsDocument your assumptions

List the strategiesList the strategies Think Business not OracleThink Business not Oracle

Benchmark your ideasBenchmark your ideas


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The MythsThe Myths

Separating Data and Indexes improvesSeparating Data and Indexes improvesperformanceperformance

A High cache hit ratio indicates goodA High cache hit ratio indicates good

performanceperformance Use the most selective column first in a multiUse the most selective column first in a multi--

column indexcolumn index

Store objects in as few extents as possible,Store objects in as few extents as possible,

preferably onepreferably one Set PCTIncrease to zero for tables and 1% forSet PCTIncrease to zero for tables and 1% for

tablespacestablespaces


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Why do we believe all this?Why do we believe all this?

Myths almost all have a basis in factMyths almost all have a basis in fact

But those facts may no longer be validBut those facts may no longer be valid

We dont test.We dont test. We believe experts.We believe experts.


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Separate Data and IndexesSeparate Data and Indexes --11

The basisThe basis io distributionio distribution

Serial accessSerial access

Multiple usersMultiple users SMONSMON


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Separate Data and IndexesSeparate Data and Indexes --22

FilesystemsFilesystems

LVM and Cached ControllersLVM and Cached Controllers

Parent/ChildParent/Child Stripe your io across devicesStripe your io across devices


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High cache hit ratio is goodHigh cache hit ratio is good

The basisThe basis physical reads are expensivephysical reads are expensiveso minimise themso minimise them

So is logical ioSo is logical ioAn ExampleAn Example

Go_faster.sqlGo_faster.sql

How to calculate BCHR.How to calculate BCHR.Buffer is pinned, sort direct writes?Buffer is pinned, sort direct writes?

Minimise your io. Period.Minimise your io. Period.


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Use the most selective column firstUse the most selective column first

The basisThe basis the CBO will use the selectivitythe CBO will use the selectivityof the column to determine the usefulnessof the column to determine the usefulness

of the indexof the index It isnt as simple as thatIt isnt as simple as that

Compressible indexesCompressible indexes

IndexSkip

ScanIndex

Skip

Scan

Work with the optimiser but know what itWork with the optimiser but know what itdoes.does.


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Know what the Optimiser DoesKnow what the Optimiser Does

RBO OperationRBO Operation

Myths about CBOMyths about CBO

10053 Trace10053 Trace ((resultsresults)) Brietling 2002Brietling 2002

SeminarSeminar


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Store objects in one extentStore objects in one extent

The basisThe basis unknown.unknown.

There is a strong argument for arrangingThere is a strong argument for arranging

objects into tablespaces based on uniformobjects into tablespaces based on uniformextent sizesextent sizes

You cant do it anyway.You cant do it anyway.

Packaged appsPackaged apps Store in LMT tablespaces of appropriateStore in LMT tablespaces of appropriate

size extentssize extents


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Set PCTIncreaseSet PCTIncrease

The basisThe basis avoid fragmentationavoid fragmentation

Packaged appsPackaged apps

Temp tablesTemp tables DBA/dev oversightDBA/dev oversight

Symptom/Problem confusionSymptom/Problem confusion

A better way LMTsA better way LMTs


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The way forwardThe way forward

Rules of thumb are second bestRules of thumb are second best

State the assumptions for your designsState the assumptions for your designs

Test themTest them Refine the test for new releasesRefine the test for new releases

Be ScepticalBe Sceptical


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CreditsCredits

Connor McDonaldConnor McDonald Choose_a_hit_ratio.sqlChoose_a_hit_ratio.sqlwww.oracledba.co.ukwww.oracledba.co.uk

Jonathan LewisJonathan Lewis how calculations of BCHR are wrong.how calculations of BCHR are wrong.http://www.jlcomp.demon.co.ukhttp://www.jlcomp.demon.co.uk

Howard RogersHoward Rogers Notes on separate data/indexesNotes on separate data/indexeshttp://groups.google.comhttp://groups.google.com

Tom KyteTom Kyte Compressible indexes and Index Skip ScanCompressible indexes and Index Skip Scanhttp://asktom.oracle.comhttp://asktom.oracle.com

Julian DykeJulian Dyke Notes on compressible indexesNotes on compressible indexes Wolfgang BreitlingWolfgang Breitling 10053 trace and what it means.10053 trace and what it means.

www.hotsos.comwww.hotsos.com

My SiteMy Site near emptynear empty www.niall.litchfield.dial.pipex.comwww.niall.litchfield.dial.pipex.com


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Over to youOver to you

5 great oracle myths

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