This helps to distribute the I/O required for a given
operation, and it reduces the possibility that fragmentation may occur in other tablespaces
due to the allocation and deallocation of temporary segments. When the database is created,
a default temporary tablespace can be created for any new users who do not have a specific
temporary tablespace assigned; if the SYSTEM tablespace is locally managed, a separate temporary
tablespace must be created to hold temporary segments.
How space is managed within a segment depends on how the tablespace containing the block
is created. If the tablespace is dictionary managed, the segment uses freelists to manage space
within the data blocks; if the tablespace is locally managed, space in segments can be managed
with either freelists or bitmaps. Oracle strongly recommends that all new tablespaces be created
as locally managed and that free space within segments be managed automatically with bitmaps.
Automatic segment space management allows more concurrent access to the bitmap lists in a
segment compared to freelists; in addition, tables that have widely varying row sizes make more
efficient use of space in segments that are automatically managed.
As I mentioned earlier, in the section titled ???Data Blocks,??? if a segment is created with automatic
segment space management, bitmaps are used to manage the space within the segment. As a result,
the pctused, freelist, and freelist groups keywords within a create table or create index statement
are ignored.
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