Database Reference
In-Depth Information
schemas. Segments are physically broken up into extents. An
extent
is a
number of blocks added to a segment (datafile) when an object requires
more space.
Figure 3.10 shows the different types of segments, synonymous from
this perspective with a tablespace. In other words, in terms of functionality,
a rollback segment accomplishes the same objective as a rollback tablespace,
where rollback segments are created within rollback tablespaces.
Note:
Rollback segments are generally referred to as undo or automated
undo segments in Oracle Database 10
g
. Manual rollback is deprecated.
The SYSTEM tablespace contains Oracle system metadata, or the data
about the data. Metadata contains table structures including column
names, lengths, and datatypes, among many other things. The SYSAUX
tablespace is new to Oracle Database 10
g
, containing various tool and util-
ity objects such as for Recovery Manager (RMAN). Generally, data (DATA)
and index (INDEX) tablespaces are split as shown in Figure 3.10 because
tables and indexes are usually scanned at the same time. The UNDO
tablespace contains data for undoing of transactions already executed plus
providing flashback capability. The TEMP tablespace is specially formatted
for fast on-disk sorting when memory capacity is exceeded for a sort. Tem-
porary sort space is used for other functions as well, such as consistent
exports. Partition tablespaces are both logically and physically split into
chunks, where separate chunks can be accessed individually or as multiple
groups executed in parallel.
The Controlfile, redo, and archive logs shown in Figure 3.10 are
described in the next section.
3.3.2
Controlfiles, Logging, and Archiving
Looking once again at Figure 3.10, there are arrows pointing from the Con-
trolfile to the redo logs and the archive logs. Redo logs and archive logs con-
tain records of past database changes; recoverability is provided by
Controlfile pointers to the redo logs and archive logs. When recovery is
attempted, changes are read from log files and applied to restored datafiles.
The Controlfile also contains pointers to datafiles. If datafile pointers in the
datafiles are behind the Controlfile pointers for the datafiles, then the data-
files can be recovered using log file entries. When datafiles are up to date,
pointers in the Controlfile, to the datafiles, and values in datafiles will be
