Undo segment management is discussed in detail in Chapter 7, whereas the tools to help size
the undo tablespaces are discussed later in this chapter.
Fragmented Tablespaces and Segments
As of Oracle8i, a tablespace that is locally managed uses bitmaps to keep track of free space, which,
in addition to eliminating the contention on the data dictionary, eliminates wasted space because
all extents are either the same size (with uniform extent allocation) or are multiples of the smallest
size (with autoallocation). For migrating from a dictionary-managed tablespace, we will review
an example that converts a dictionary-managed tablespace to a locally managed tablespace. In a
default installation of Oracle Database 10g or Oracle Database 11g using the Database Creation
Assistant (DBCA), all tablespaces, including the SYSTEM and SYSAUX tablespaces, are created as
locally managed tablespaces.
Even though locally managed tablespaces with automatic extent management (using the
autoallocate clause) are created by default when you use create tablespace, you still need to
specify extent management local if you need to specify uniform for the extent management
type in the create tablespace statement:
SQL> create tablespace USERS4
2 datafile '+DATA'
3 size 250M autoextend on next 250M maxsize 2000M
4 extent management local uniform size 8M
5 segment space management auto;
Tablespace created.
This tablespace will be created with an initial size of 250MB, and it can grow as large
as 2000MB (2GB); extents will be locally managed with a bitmap, and every extent in this
tablespace will be exactly 8MB in size.
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