Oracle Database Concepts - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 91

Things to Remember Before Moving Ahead
1) Figure 3-2: Physical Architecture of Oracle database.
Oracle database is composed of mainly three different
types of files. Oracle Instance updates the control file
whenever the physical structure of database is changed
by DBA. Data Files holds the actual data and if you
keeping added records to your database tables, the sizes
of these data files increase along with the size of redo
Log files. Redo Log files get utilized in a circular
fashion. It is recommended that Oracle should be run
under “Archived Log Mode” so that Redo Log files get
copied before getting overwritten. Redo Log files along
with Archived Redo Log files plays a major role in the
database recovery process.
2) Figure 3-4: Logical architecture of Oracle database.
Tablespace is like small departmental level database.
Logically database is composed of tablespaces. Each
table space can be visualized as a department level
database e.g. for each department of a company you
would be having one tablespace. Each tablespace is
composed of segments and Oracle allots a segment each
time you create a database object e.g. tables, views etc
using CREATE statement. Each Segment is composed of
Extents and extents are nothing but contiguous DB
Blocks. DB Block is usually the even multiple of OS
Block and its value is specified in parameter file
(init.ora).
3) Figure 3-7: Relationship between Physical and Logical
architecture of Oracle database. One object (e.g. table)
can be in two or more data files but only under one
tablespace. Each tablespace can have one or more data
files associated with it but each data file can never be
associated with more than one table space. Segments
can be sitting under two or more data files under the
same tablespace. Tablespace is nothing but just an alias
given to the group of data files or data file.