99

Storage Structure and Relationships

Storage Structure and Relationships

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ObjectivesObjectives

• Listing the different segment types and their uses

• Controlling the use of extents by segments

• Stating the use of block space utilization parameters by objects

• Obtaining information about storage structures from the data dictionary

• Locating the segments by considering fragmentation and life-spans

• Listing the different segment types and their uses

• Controlling the use of extents by segments

• Stating the use of block space utilization parameters by objects

• Obtaining information about storage structures from the data dictionary

• Locating the segments by considering fragmentation and life-spans

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Database

Logical Physical

Tablespace Data file

O/S blockOracle block

Segment

Extent

Database Storage HierarchyDatabase Storage Hierarchy

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Types of SegmentsTypes of Segments

TableTable

ClusterCluster

Table Table partitionpartition

IndexIndex

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Types of SegmentsTypes of SegmentsSegments occupy spaceSegments occupy space

•TableTable

•Table PartitionTable Partition

table with high concurrent usagetable with high concurrent usage

need partitioning optionneed partitioning option

•ClusterCluster

based on key col valuesbased on key col values

one or more tables in a clusterone or more tables in a cluster

•IndexIndex

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Types of SegmentsTypes of Segments

Index-organizedIndex-organizedtabletable Index Index

partitionpartition

RollbackRollbacksegmentsegment TemporaryTemporary

segmentsegment

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Types of SegmentsTypes of Segments

•Index-organized tableIndex-organized tableindex+data stored togetherindex+data stored together

•Index PartitionIndex Partitionindex can be partitionedindex can be partitioned

•Rollback segmentRollback segmentbefore image of a data valuebefore image of a data value

•Temporary segmentTemporary segment

when sorts need more spacewhen sorts need more space

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Types of SegmentsTypes of Segments

LOB index LOB index LOBLOB

segmentsegment

BootstrapBootstrapsegmentsegment

Nested tableNested table

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Storage Clause PrecedenceStorage Clause Precedence

Oracle default (5 blocks)

Tablespace

Segment

Controls how extents are allocated to segmentsControls how extents are allocated to segments

Storage Storage parameter at parameter at segment level segment level overrides the overrides the one at TS level, one at TS level, etc.etc.

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Extent Allocation and DeallocationExtent Allocation and Deallocation

• Allocated when the segment is+ Created + Extended+ Altered

• Deallocated when the segment is– Dropped– Altered– Truncated– Automatically resized (rollback segments

only)

• Allocated when the segment is+ Created + Extended+ Altered

• Deallocated when the segment is– Dropped– Altered– Truncated– Automatically resized (rollback segments

only)

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Used and Free ExtentsUsed and Free Extents*When tablespace is created, the data files *When tablespace is created, the data files contain the following elements:contain the following elements:-A header block: the first blockA header block: the first block-One free extent consisting of the One free extent consisting of the remaining part of the fileremaining part of the file*As segments are created, or extended, *As segments are created, or extended, they are allocated space from the pool of they are allocated space from the pool of free extents; as segments are released, free extents; as segments are released, truncated, released extents are added to truncated, released extents are added to the poolthe pool*Frequent allocation/deallocation leads to *Frequent allocation/deallocation leads to fragmentationfragmentation

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Used and Free ExtentsUsed and Free Extents

Data fileData file

Free extentFree extentUsed extentUsed extentFile headerFile header

SMON compacts free space; this is SMON compacts free space; this is called coalescingcalled coalescing

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Coalescing Free SpaceCoalescing Free Space

Free extentFree extentUsed extentUsed extentFile headerFile header

BeforeBefore

ALTER TABLESPACE data01 COALESCE;ALTER TABLESPACE data01 COALESCE;

AfterAfter

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Database Block: ReviewDatabase Block: Review

• Minimum unit of I/O

• Consists of one or more O/S blocks

• Set by DB_BLOCK_SIZE

• Set at database creation and cannot be altered

• Minimum unit of I/O

• Consists of one or more O/S blocks

• Set by DB_BLOCK_SIZE

• Set at database creation and cannot be altered

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Database Block ContentsDatabase Block Contents

Header Header (contains (contains row ptrs, trans info,row ptrs, trans info,Can grow as more rows areCan grow as more rows areadded)added)

Free spaceFree space(can get fragmented as(can get fragmented asRows are inserted andRows are inserted andDeleted; needs coalescing)Deleted; needs coalescing)

DataData

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Table Row ContentsTable Row Contents

Row headerRow header

(locked or not, deleted or not, trans., etc.)(locked or not, deleted or not, trans., etc.)

Col 1 size, col1 dataCol 1 size, col1 data

Col k sizeCol k size

(null)(null)

Col l sizeCol l size

(null)(null)

Define cols that can take null values lastDefine cols that can take null values last

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Block Space Utilization Parameters(managing free space to account for updates to rows)

Block Space Utilization Parameters(managing free space to account for updates to rows)

INITRANSINITRANSMAXTRANS MAXTRANS (initial and max num of(initial and max num oftransactions that can concurrently make transactions that can concurrently make changes to the block at a point in time)changes to the block at a point in time)

PCTFREE PCTFREE (can insert rows until(can insert rows untilfree space falls below this; default is 10)free space falls below this; default is 10)

PCTUSED (PCTUSED (if free space falls if free space falls below this, block goes on freebelow this, block goes on freeList; default is 40)List; default is 40)

Blocks are on “free” listBlocks are on “free” list

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Block Space UsageBlock Space Usage

InsertsInserts

InsertsInserts

InsertsInserts

InsertsInserts

1 2

3 4

PCTFREE=20PCTFREE=20 PCTUSED=40PCTUSED=40

80%80%

80%80%

40%40%

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ExampleExample

Create table Employee

(EID number not null,

ENAME varchar2(50)

Using INDEX Tablespace INDX

Pctfree 10 Pctused 40 INITRANS 5

STORAGE (INITIAL … PCTINCREASE)

)

PCTFREE 10 PCTUSED 60 INITRANS 5

STORAGE (INITIAL….)

TABLESPACE DATA01;

-- COLUMN placement can also be specified

Create table Employee

(EID number not null,

ENAME varchar2(50)

Using INDEX Tablespace INDX

Pctfree 10 Pctused 40 INITRANS 5

STORAGE (INITIAL … PCTINCREASE)

)

PCTFREE 10 PCTUSED 60 INITRANS 5

STORAGE (INITIAL….)

TABLESPACE DATA01;

-- COLUMN placement can also be specified

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Segment Space Management in LMT Segment Space Management in LMT

From Oracle 9i, one can not only have bitmap managed tablespaces, but also bitmap managed segments when setting Segment Space Management to AUTO for a tablespace. Look at this example:

SQL> CREATE TABLESPACE ts4 DATAFILE '/oradata/ts4_01.dbf' SIZE 50M EXTENT MANAGEMENT LOCAL SEGMENT SPACE MANAGEMENT AUTO;

From Oracle 9i, one can not only have bitmap managed tablespaces, but also bitmap managed segments when setting Segment Space Management to AUTO for a tablespace. Look at this example:

SQL> CREATE TABLESPACE ts4 DATAFILE '/oradata/ts4_01.dbf' SIZE 50M EXTENT MANAGEMENT LOCAL SEGMENT SPACE MANAGEMENT AUTO;

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Segment Space Management in LMT Segment Space Management in LMT

Segment Space Management eliminates the need to specify and tune the PCTUSED, FREELISTS, and FREELISTS GROUPS storage parameters for schema objects. The Automatic Segment Space Management feature improves the performance of concurrent DML operations significantly since different parts of the bitmap can be used simultaneously eliminating serialization for free space lookups against the FREELSITS. This is of particular importance when using RAC.

Segment Space Management eliminates the need to specify and tune the PCTUSED, FREELISTS, and FREELISTS GROUPS storage parameters for schema objects. The Automatic Segment Space Management feature improves the performance of concurrent DML operations significantly since different parts of the bitmap can be used simultaneously eliminating serialization for free space lookups against the FREELSITS. This is of particular importance when using RAC.

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Data Dictionary ViewsData Dictionary Views

TablespacesDBA_TABLESPACES

SegmentsDBA_SEGMENTS

Data filesDBA_DATA_FILES

Free extentsDBA_FREE_SPACE

Used extentsDBA_EXTENTS

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Querying Segment InformationQuerying Segment Information

DBA_SEGMENTS

– General information– OWNER

SEGMENT_NAME

– SEGMENT_TYPE

– TABLESPACE_NAME

DBA_SEGMENTS

– General information– OWNER

SEGMENT_NAME

– SEGMENT_TYPE

– TABLESPACE_NAME

– Size– EXTENTS

– BLOCKS

– Storage settings– INITIAL_EXTENT

– NEXT_EXTENT

– MIN_EXTENTS

– MAX_EXTENTS

– PCT_INCREASE

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ExampleExample

Select segment_name, tablespace_name,

extents, blocks

From dba_segments

Where owner = ‘SCOTT’;

Select segment_name, tablespace_name,

extents, blocks

From dba_segments

Where owner = ‘SCOTT’;

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Getting Used Extent InformationGetting Used Extent Information

DBA_EXTENTS

– Identification– OWNER

– SEGMENT_NAME

– EXTENT_ID

– Location and size– TABLESPACE_NAME

– RELATIVE_FNO

– FILE_ID

– BLOCK_ID

– BLOCKS

DBA_EXTENTS

– Identification– OWNER

– SEGMENT_NAME

– EXTENT_ID

– Location and size– TABLESPACE_NAME

– RELATIVE_FNO

– FILE_ID

– BLOCK_ID

– BLOCKS

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ExampleExample

Select extent_id, file_id, block_id, blocks

From dba_extents

Where owner = ‘SCOTT’ AND

segment_name = ‘EMP’;

Select extent_id, file_id, block_id, blocks

From dba_extents

Where owner = ‘SCOTT’ AND

segment_name = ‘EMP’;

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Checking Free Extent Information

Checking Free Extent Information

DBA_FREE_SPACE

– Location and size– TABLESPACE_NAME

– RELATIVE_FNO

– FILE_ID

– BLOCK_ID

– BLOCKS

DBA_FREE_SPACE

– Location and size– TABLESPACE_NAME

– RELATIVE_FNO

– FILE_ID

– BLOCK_ID

– BLOCKS

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ExampleExample

Select tablespace_name, count(*),

max(blocks), sum(blocks)

From dba_free_space

Group by tablespace_name;

Select tablespace_name, count(*),

max(blocks), sum(blocks)

From dba_free_space

Group by tablespace_name;

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Organizing Tablespaces Based on Fragmentation Propensity

Organizing Tablespaces Based on Fragmentation Propensity

Tablespace

SYSTEM

TOOLS

DATAn

INDEXn

RBSn

TEMPn

Fragmentation

Zero

Very low

Low

Low

High

Very high*

Usage

Data dictionary

Applications

Data segments

Index segments

Rollback segments

Temporary segments* Relevant only if tablespace PERMANENT* Relevant only if tablespace PERMANENT

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SummarySummary

• Allocation and deallocation of extents for different types of segments

• Block space utilization parameters

• Allocation and deallocation of extents for different types of segments

• Block space utilization parameters