An extent is a logical unit of database storage space allocation made up of contiguous data blocks. Data blocks in an extent are logically contiguous but can be physically spread out on disk because of RAID striping and file system implementations.
Allocation of Extents
By default, the database allocates an initial extent for a data segment when the segment is created. An extent is always contained in one data file.
Although no data has been added to the segment, the data blocks in the initial extent are reserved for this segment exclusively. The first data block of every segment contains a directory
of the extents in the segment. Figure 12–16 shows the initial extent in a segment in a data file that previously contained no data.
If the initial extent become full, and if more space is required, then the database automatically allocates an incremental
extent for this segment. An incremental extent is a subsequent extent created for the segment.
The allocation algorithm depends on whether the tablespace is locally managed or dictionary-managed. In the locally managed case, the database searches the bitmap of a data file for
adjacent free blocks. If the data file has insufficient space, then the database looks in another data file. Extents for a segment are always in the same tablespace but may be in different data files.
Figure 12–17 shows that the database can allocate extents for a segment in any data file in the tablespace. For example, the segment can allocate the initial extent in users01.dbf, allocate
the first incremental extent in users02.dbf, and allocate the next extent in users01.dbf.
The blocks of a newly allocated extent, although they were free, may not be empty of old data. In ASSM, Oracle Database formats
the blocks of a newly allocated extent when it starts using the extent, but only as needed (see "Segment Space and the High Water Mark" on page 12-27).
Deallocation of Extents
In general, the extents of a user segment do not return to the tablespace unless you drop the object using a DROP command.
In Oracle Database 11g Release 2 (11.2.0.2), you can also drop the segment using the DBMS_SPACE_ADMIN package. For example, if you delete all rows in a table, then the database does not reclaim the data blocks for use by other objects in the tablespace.
In some circumstances, you can manually deallocate space. The Oracle Segment Advisor helps determine whether an object has
space available for reclamation based on the level of fragmentation in the object. The following techniques can free extents:
■You can use an online segment shrink to reclaim fragmented space in a segment. Segment shrink is an online, in-place operation. In general, data compaction leads to better cache utilization
and requires fewer blocks to be read in a full table scan.
■You can move the data of a nonpartitioned table or table partition into a new segment, and optionally into a different tablespace for which you have quota.
■You can rebuild or coalesce the index (see "Reuse of Index Space" on page 12-14).
■You can truncate a table or table cluster, which removes all rows. By default, Oracle Database deallocates all space used by the removed rows except that specified by the MINEXTENTS
storage parameter. In Oracle Database 11g Release 2 (11.2.0.2), you can also use TRUNCATE with the DROP ALL STORAGE option to drop entire segments.
■You can deallocate unused space, which frees the unused space at the high water mark end of the database segment and makes the space available for other segments in the tablespace (see
"Segment Space and the High Water Mark" on page 12-27).
When extents are freed, Oracle Database modifies the bitmap in the data file for locally managed tablespaces to reflect the regained extents as available space. Any data in the blocks of freed extents
becomes inaccessible.
Storage Parameters for Extents
Every segment is defined by storage parameters expressed in terms of extents. These parameters control how Oracle Database
allocates free space for a segment.
The storage settings are determined in the following order of precedence, with setting higher on the list overriding settings lower on the list:
1.Segment storage clause
2.Tablespace storage clause
3.Oracle Database default
A locally managed tablespace can have either uniform extent sizes or variable extent sizes determined automatically by the system:
■For uniform extents, you can specify an extent size or use the default size of 1 MB. All extents in the tablespace are of this size. Locally managed temporary tablespaces can only use
this type of allocation.
■For automatically allocated extents, Oracle Database determines the optimal size of additional extents.
For locally managed tablespaces, some storage parameters cannot be specified at the tablespace level. However, you can specify
these parameters at the segment level. In this case, the databases uses all parameters together to compute the initial size of the segment. Internal algorithms determine the subsequent size of each extent.
