Databases Reference
In-Depth Information
different sizes. Typically, one or more rows of a table will reside in a database
block, although very long rows may span several database blocks.
Extents group together logically contiguous database blocks in a table-
space. All database blocks within a single extent will store the same kind of
information.
A database block can have a size of 2KB, 4KB, 8KB, 16KB, or 32KB. Once
any tablespace, including the
Oracle block
See
database block.
tablespaces, is created with a
given block size, it cannot be changed. If you want the tablespace to have a larger
or smaller block size, you need to create a new tablespace with the new block
size, move the objects from the old tablespace to the new tablespace, and then
drop the old tablespace.
SYSTEM
and
SYSAUX
Schemas
A
is another logical structure that can classify or group database
objects. A schema has a one-to-one correspondence with a user account in the
Oracle database, although some schemas may be designed to hold only objects
that may be referenced by other database users. For instance, in the logical
structure illustration, Segments 1 and 3 may be owned by the
schema
schema
A named group of objects associated
with a particular user account, such as
tables, indexes, functions, and so forth.
HR
schema, while
Segment 2 may be owned by the
schema.
A schema is not directly related to a tablespace or any other logical storage
structure; the objects that belong to a schema may be in many different
tablespaces. Conversely, a tablespace may hold objects for many different sche-
mas. A schema is a good way to group objects in the database for purposes of
security and access control.
SCOTT
Physical Storage Structures
From the perspective of building queries and running reports, regular users
don't need to know much about the underlying physical structure of the
database on disk. However, DBAs do need to understand these database
components.
of the Oracle database consists of datafiles, redo
log files, and control files. On a day-to-day basis, the DBA will deal most
often with the datafiles, since this is where all of the user and system objects,
such as tables and indexes, are stored. The illustration below shows the phys-
ical structure and its relationship to the Oracle memory structures and logical
storage structures.
The
physical structure
physical structures
Structures of an Oracle database, such
as datafiles on disk, that are not directly
manipulated by users of the database.
Physical structures exist at the operating
system level.
