Permanent tablespaces contain segments that persist beyond the duration of a session or a transaction.
Although the undo tablespace may have segments that are retained beyond the end of a session
or a transaction, it provides read consistency for select statements that access tables being modified
as well as provides undo data for a number of the flashback features of the database. Primarily,
however, undo segments store the previous values of columns being updated or deleted, or to
provide an indication that the row did not exist for an insert so that if a user??™s session fails before the
user issues a commit or a rollback, the updates, inserts, and deletes will be removed. Undo segments
are never directly accessible by a user session, and undo tablespaces may only have undo segments.
As the name implies, temporary tablespaces contain transient data that exists only for the
duration of the session, such as space to complete a sort operation that will not fit in memory.
Bigfile tablespaces can be used for any of these three types of tablespaces, and they simplify
tablespace management by moving the maintenance point from the datafile to the tablespace.
Bigfile tablespaces consist of one and only one datafile. There are a couple of downsides to bigfile
tablespaces, however, and I will present these later in this chapter.
Permanent
The SYSTEM and SYSAUX tablespaces are two examples of permanent tablespaces. In addition,
any segments that need to be retained by a user or an application beyond the boundaries of a
session or transaction should be stored in a permanent tablespace.
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