Locally Managed Tablespaces
Prior to Oracle8i, there was only one way to manage free space within a tablespace??”by using
data dictionary tables in the SYSTEM tablespace. If a lot of insert, delete, and update activity
occurs anywhere in the database, there is the potential for a ???hot spot??? to occur in the SYSTEM
tablespace where the space management occurs. Oracle removed this potential bottleneck by
introducing locally managed tablespaces (LMTs). A locally managed tablespace tracks free space
in the tablespace with bitmaps, as discussed in Chapter 1. These bitmaps can be managed very
efficiently because they are very compact compared to a freelist of available blocks. Because they
are stored within the tablespace itself, instead of in the data dictionary tables, contention in the
SYSTEM tablespace is reduced.
As of Oracle 10g, by default, all tablespaces are created as locally managed tablespaces,
including the SYSTEM and SYSAUX tablespaces. When the SYSTEM tablespace is locally
managed, you can no longer create any dictionary-managed tablespaces in the database that
are read/write. A dictionary-managed tablespace may still be plugged into the database from
an earlier version of Oracle, but it is read-only.
An LMT can have objects with one of two types of extents: automatically sized or all of a
uniform size. If extent allocation is set to UNIFORM when the LMT is created, all extents, as
expected, are the same size.
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