Extents
The extent is the next level of logical grouping in the database. An extent consists of one or more
database blocks. When you enlarge a database object, the space added to the object is allocated
as an extent.
Segments
The next level of logical grouping in a database is the segment. A segment is a group of extents
that form a database object that Oracle treats as a unit, such as a table or index. As a result, this is
typically the smallest unit of storage that an end user of the database will deal with. Four types of
segments are found in an Oracle database: data segments, index segments, temporary segments,
and rollback segments.
Data Segment
Every table in the database resides in a single data segment, consisting of one or more extents;
Oracle allocates more than one segment for a table if it is a partitioned table or a clustered table .
I discuss partitioned and clustered tables later in this chapter. Data segments include LOB (large
object) segments that store LOB data referenced by a LOB locator column in a table segment (if
the LOB is not stored inline in the table).
Index Segment
Each index is stored in its own index segment. As with partitioned tables, each partition of a
partitioned index is stored in its own segment. Included in this category are LOB index segments;
a table??™s non-LOB columns, a table??™s LOB columns, and the LOBs??™ associated indexes can all
reside in their own tablespace to improve performance.
Pages:
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61