Oracle Segment - (Free) Space Management (ASSM) of a Tablespace

About

In a locally managed tablespace, there are two methods that Oracle Database can use to manage segment space:

• automatic
• and manual.

To manage free space in the segment:

• Manual segment space management uses linked lists called “freelists”
• Automatic segment space management uses bitmaps.

Automatic segment space management is the more efficient method, and is the default for all new permanent, locally managed tablespaces.

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Maintenance

Default

Automatic segment space management is the default for all new permanent, locally managed tablespaces.

Create

• Automatic (ASSM)

CREATE TABLESPACE lmtbsb DATAFILE '/u02/oracle/data/lmtbsb01.dbf' SIZE 50M
    EXTENT MANAGEMENT LOCAL 
    SEGMENT SPACE MANAGEMENT AUTO; -- ASSM is enabled

• Manual

CREATE TABLESPACE lmtbsb DATAFILE '/u02/oracle/data/lmtbsb01.dbf' SIZE 50M
    EXTENT MANAGEMENT LOCAL 
    SEGMENT SPACE MANAGEMENT MANUAL; -- ASSM is disbaled

Modify

You cannot change the segment space management mode of a tablespace.

View

select tablespace_name, segment_space_management from dba_tablespaces;

Documentation / Reference

• Tablespace