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Automatic Segment Space Management:

Space management is amongst the most critical issues in databases. It is a simple and efficient way of managing space within a segment. Automatic segment-space management enables database administrators to completely eliminate the need to specify and tune the pctused, freelists and freelist groups storage parameters for schema objects created in the tablespace. Even if any of these attributes are specified, they are ignored.

Specifying Space:

When a locally managed tablespace using the create TABLESPACE statement is created, the SEGMENT SPACE MANAGEMENT clause lets you specify how free and used space within a segment is to be managed.

Creating tablespace with Automatic segment space management:

Below statement creates tablespace mytbs1 with automatic segment-space management

CREATE TABLESPACE mytbs1
DATAFILE '/u01/oracle/data/mytbs01.dbf' SIZE 500M
EXTENT MANAGEMENT LOCAL
SEGMENT SPACE MANAGEMENT AUTO;

When an object such as a table or index is created using the locally managed tablespace, with automatic segment-space management enabled, there is no need to specify the pctfree or freelists .

In-segment free/used space:

The in-segment free/used space is tracked using bitmaps as opposed to the free lists. When you cannot use the locally managed tablespace, and therefore the automatic management space feature, you have to depend on the traditional method of managing free lists and free lists groups.

Administrative ease of use:

Automatic segment-space management provides administrative ease of use by avoiding the specification of storage parameters.

Object Handling:

Automatic segment-space management is a good method for handling objects with varying row sizes.

Run-time adjustment:

Automatic segment-space management provides better run-time adjustment for variations in concurrent access and avoids tedious tuning methods.

Multi-instance behavior:

Automatic segment-space management provides better multi-instance behavior in terms of performance/space utilization.

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