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.

Articles Related

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;

TABLESPACE_NAME	SEGMENT_SPACE_MANAGEMENT
SYSTEM	MANUAL
SYSAUX	AUTO
UNDOTBS	MANUAL
TEMP	MANUAL
USERS	AUTO

Documentation / Reference

Tablespace