Primary Key : Concept

A relational database is based on the concept that the user that
queries the database needs not remember the physical location of
the record that he/she needs access to.

the user only needs to remember the table name and
the value that the user associates to.

that's why a relational database must contain a unique identifier
for each record that exists in the relational database.

thus the concept of primary key arises that provides a unique
identifier for each record called as the "PRIMARY KEY".

ORACLE Logical Database Structures

Tablespaces

A database is divided into logical storage units called
tablespaces,which group related logical structures together.
One or more datafiles are explicitly created for each tablespace
to physically store the data of all logical structures in a
tablespace.

Oracle Data Blocks

Oracle database data is stored in data blocks. One
data block corresponds to a specific number of bytes of
physical database space on disk.
The standard block size is specified by the DB_BLOCK_SIZE
initialization parameter.

Extents

An extent is a specific number
of contiguous data blocks, obtained in a single allocation,
used to store a specific type of information.

Segments

A segment is a set of extents allocated for a certain logical
structure.

Types Of Segments -

Data segment
Index segment
Temporary segment
Rollback segment

ORACLE Physical Database Structures

Datafiles

Every Oracle database has one or more physical datafiles.
The datafiles contain all the database data.
The data of logical database structures, such as tables and
indexes, is physically stored in the datafiles allocated for a
database.

Control Files

Every Oracle database has a control file. A control file contains
entries that specify the physical structure of the database.
For example, it contains the following information:


Database name

Names and locations of datafiles and redo log files

Time stamp of database creation


Redo Log Files

Every Oracle database has a set of two or more redo log files.
The set of redo log files is collectively known as the redo log
for the database. A redo log is made up of redo entries (also called
redo records).
The primary function of the redo log is to record all changes made
to data. If a failure prevents modified data from being permanently
written to the datafiles, then the changes can be obtained from the
redo log, so work is never lost.

Archive Log Files

You can enable automatic archiving of the redo log. Oracle
automatically archives log files when the database is in ARCHIVELOG mode.

Parameter Files

Parameter files contain a list of configuration parameters for
that instance and database.