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Quick Tips #3

Table Compression

Dos and Donts

Tim Gorman

(tim@sagelogix.com)

Principal

SageLogix, Inc.

IOUG-A Live 2003IOUG-A Live 2003

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SyntaxSyntax

Availablein Oracle9iRelease 2 (v9.2.0.x)Physicalstorageattribute fortables andmaterialized views

[ CREATE | ALTER ] TABLE

[ COMPRESS | NOCOMPRESS ]

Canbeused forRANGE orLIST partitionsButcannot beusedwith HASH partitions

Butcannot beused for HASH orLISTsub-partitions

Canbespecified for NESTED tablesButcannot beusedwithanyLOBconstruct

SuchasCLOB,BLOB,BFILE,and VARRAY

Notvalid forindex-organizedorexternaltables

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SyntaxSyntax

Canbesetasa DEFAULTattributeontablespaces

Affectsbothtablesandindexesinthetablespace

Unlessotherwisespecified

[ CREATE | ALTER ] TABLESPACE

DEFAULT [ COMPRESS | NOCOMPRESS ]

STORAGE

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Howdoesitwork?Howdoesitwork?

Storing repeateddatavaluesonce ineachblockA symbol table ofdatavaluescreatedineachblock

Thesymboltableisstoredasanother table intheblock

Eachcolumnina rowinablock referencesback toanentryinthesymboltableintheblock

Header &

Tailer

Table &

Column Map

ITL

Symbol table

Free

Row data

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Howdoesitwork?Howdoesitwork?

Allactualdatavaluesarestoredintheblockssymbol table

Eachcolumnwithineach row referencestheentryinthe

symbol table withthedatavalue

Data-valuedistributioniscrucial!Manydistinctandnon-repeatingdatavaluesdonot

achievegoodcompression

Repeatingdatavaluesachieveexcellentcompression

Know thine data

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Impacton DMLImpacton DML

Bulk-loadingINSERToperations performcompression

CREATE TABLE ASSELECT

INSERT /*+ APPEND */ (single-threadedand parallel)

ALTERTABLE MOVE

ALTERTABLE MOVE PARTITION

ALTERTABLE MERGE PARTITION

ALTERTABLE SPLITPARTITION

SQL*Loader DIRECT=TRUE

ConventionalINSERToperationsunaffected

SELECT, UPDATE,and DELETE behavioralso

unaffected

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Impacton DMLImpacton DML

SELECTNo performancedifferencesobserved

Testingusingrepeatingandnon-repeatingdata

Theoretically,some performanceimprovementshouldbeexpected dueto performingless I/O

FULLtablescansshouldbenefitmoreIndexedscansshouldnotbenefitatall

probable performancedecrease

INSERTUp to 6-8x performancedegradationobserved

DuringtestingwithrepeatingdataOnly 2-3x performancedegradationobservedwithnon-repeatingdata

Theoretically,some performanceimprovementshouldbeexpected dueto performingless I/O

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Impacton DMLImpacton DML

UPDATEUp to 6-8xnegative performanceimpactobserved

Only whenupdatingrepeatingdatavalues

Zero performanceimpactobservedupdatingnon-

repeatingdatavalues

Timingsandlogical-readsconsistentwith NOCOMPRESS

scenarios

DELETE

About 2xnegative performanceimpactobserved

Only whendeletingrepeatingdatavalues

Zero performanceimpactobserveddeletingnon-

repeatingdatavalues

Timingsandlogical-readsconsistentwith NOCOMPRESS

scenarios

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UsageScenariosUsageScenarios

Alteringatabletocompressnewlyinsertedblocks

SQL> alter table sales compress;

Table altered.

Sowhatwouldhappenhere?

Woulddatainallexistingblocksbecompressed?

Woulddatain previously-unusedblocksbecompressed,

fromnowon?Arethereanyotherimplications?

Indexes?

Dependentcompiledobjects?

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UsageScenariosUsageScenarios

Movingatabletocompressall rows

SQL> alter table sales move compress;

Table altered.

Sowhatwouldhappenhere?Woulddatainallexistingblocksbecompressed?

Woulddatain previously-unusedblocksbecompressed,fromnowon?

Arethereanyotherimplications?Indexes?

Dependentcompiledobjects?

Dont forget DBMS_REDEFINITION asanalternative

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UsageScenariosUsageScenarios

Compressingandmoving partitionsbeforesettinga

tablespacetoREAD ONLY

SQL> alter table sales

2 move partition p1998013 tablespace ro_p1998q1

4 compress nologging parallel;

Table altered.

SQL> alter tablespace ro_p1998q1 read only;

Tablespace altered.

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UsageScenariosUsageScenarios

Compressingandmerging partitionsbeforesettingatablespacetoREAD ONLY

SQL> alter table sales

2 merge partition p199801, p199802, p1998033 into partition p1998q1

4 tablespace ro_p1998q1

5 compress nologging parallel;

Table altered.

SQL> alter tablespace ro_p1998q1 read only;

Tablespace altered.

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WarningsWarnings

Local partitionedindexesaremarkedUNUSABLE duringcompression

Includesindexesonnon-partitionedtables

Mustbe rebuilt

Global partitionedindexescanbemaintainedusing UPDATE GLOBAL

INDEXESIncludesnon-partitionedindexeson partitionedtables

A rare situation when GLOBAL indexes can be morehighly available than LOCAL indexes!

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SummarySummary

Summary:For read-mostlyobjects,compressioncanhavespace

consumptionbenefits

Glitches:

NonedocumentedinMetaLink asof April 2003

Documentation:NonedocumentedinMetaLink asof April 2003

OTN hasaverygood,detailedwhite paperontable

compression

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Q&AQ&A

Quick Tips #3

Slides downloadable from

http://www.EvDBT.com/papers.htmAnd

http://www.SageLogix.com

Tim Gorman