Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

340
CHAPTER 10 ■ DATABASE TABLES
• Table partition or subpartition: This segment type is used in partitioning and is very
similar to a table segment. A table partition or subpartition segment holds just a slice of
the data from a table. A partitioned table is made up of one or more table partition segments,
and a composite partitioned table is made up of one or more table subpartition
segments.
• Index: This segment type holds an index structure.
• Index partition: Similar to a table partition, this segment type contains some slice of an
index. A partitioned index consists of one or more index partition segments.
• Lob partition, lob subpartition, lobindex, and lobsegment: The lobindex and lobsegment
segments hold the structure of a large object, or LOB. When a table containing a LOB is
partitioned, the lobsegment will be partitioned as well—the lob partition segment is
used for that. It is interesting to note that there is not a lobindex partition segment type—
for whatever reason, Oracle marks the partitioned lobindex as an index partition (one
wonders why a lobindex is given a special name!).
• Nested table: This is the segment type assigned to nested tables, a special kind of “child”
table in a master/detail relationship that we’ll discuss later.
• Rollback and Type2 undo: This is where undo data is stored. Rollback segments are
those manually created by the DBA. Type2 undo segments are automatically created
and managed by Oracle.
So, for example, a table may be a segment. An index may be a segment. I stress the words
“may be” because we can partition an index into separate segments. So, the index object itself
would just be a definition, not a physical segment—and the index would be made up of many
index partitions, and each index partition would be a segment. A table may be a segment or
not. For the same reason, we might have many table segments due to partitioning, or we might
create a table in a segment called a cluster. Here the table will reside, perhaps with other tables
in the same cluster segment.
The most common case, however, is that a table will be a segment and an index will be a
segment. That is the easiest way to think of it for now. When you create a table, you are normally
creating a new table segment and, as discussed in Chapter 3, that segment consists of
extents, and extents consist of blocks. That is the normal storage hierarchy. But it is important
to note that only the “common” case has this one-to-one relationship. For example, consider
this simple CREATE TABLE statement:
Create table t ( x int primary key, y clob, z blob );
This statement creates six segments. If you issue this CREATE TABLE statement in a schema
that owns nothing, you’ll observe the following:
ops$tkyte@ORA10G> select segment_name, segment_type
2 from user_segments;
no rows selected
ops$tkyte@ORA10G> create table t ( x int primary key, y clob, z blob );
Table created.