Logical structures overview

1.Oracle instance
In an Oracle configuration there is the base set of files, data files,

redo logs, control files, parameter files, which make up the database

and then there are one or more sets of operating system processes and

shared memory areas (each known as an SGA) that make up an Oracle

instance.

2.Tablespaces
A Database is divided into logical structures called Tablespaces. A

tablespace groups together other logical objects like Tables, Indexes

etc. Each Database is comprised of one or more Tablespaces which in

turn consists of one or more datafiles.

3.SYSTEM and SYSAUX Tablespaces
Every Oracle Database contains a SYSTEM Tablespace and a SYSAUX

Tablespace. These tablespaces are created automatically. The SYSTEM

Tablespace contains the Data Dictionary called as the Metadata of

Oracle Database. SYSAUX tablespace contains system-related tables, such

as statistics repository, that are not part of the core data

dictionary.

4.SEGMENTS
All Tablespaces, discussed above, contains segments. A segment is

associated to single tablespace, except partitioned tables and indexes,

in which each partition can be assigned to different tablespaces.

Different types of Segments are:

Table Segment
Index Segment
Temporary Segments
Undo Segments
A basic structure of an Oracle Database is a Table. Oracle 10g supports

many types of Tables

5.Index Segment
Indexes are used to speed up the access to the tables. Oracle provides

many types of Indexes.These are:B*tree Indexes Bitmap Indexes ...

6.ORACLE BLOCKS
When a datafile is created and assigned to a tablespace, Oracle formats

the file into blocks for the actual storage. The size of each Oracle

block is determined by the DB_BLOCK_SIZE initialization parameter,

which is specified at the time of creation of database. Normally,

Oracle block size is in multiples of OS Block size, which is generally

512 bytes. An Oracle block is a basic unit of storage into which all

Oracle objects store data.

Oracle block is divided into three distinct segments:

Fixed Block Header
Variable Block Header, and
Data Storage Area

Structure of an Oracle Block

Fixed Block Header
Variable Block Header
ROW1 ROW2 ROW3 …
Row 1 Data
Row 2 Data
Row 3 Data
Free Space
1.Oracle instance
In an Oracle configuration there is the base set of files, data files,

redo logs, control files, parameter files, which make up the database

and then there are one or more sets of operating system processes and

shared memory areas (each known as an SGA) that make up an Oracle

instance.

2.Tablespaces
A Database is divided into logical structures called Tablespaces. A

tablespace groups together other logical objects like Tables, Indexes

etc. Each Database is comprised of one or more Tablespaces which in

turn consists of one or more datafiles.

3.SYSTEM and SYSAUX Tablespaces
Every Oracle Database contains a SYSTEM Tablespace and a SYSAUX

Tablespace. These tablespaces are created automatically. The SYSTEM

Tablespace contains the Data Dictionary called as the Metadata of

Oracle Database. SYSAUX tablespace contains system-related tables, such

as statistics repository, that are not part of the core data

dictionary.

4.SEGMENTS
All Tablespaces, discussed above, contains segments. A segment is

associated to single tablespace, except partitioned tables and indexes,

in which each partition can be assigned to different tablespaces.

Different types of Segments are:

Table Segment
Index Segment
Temporary Segments
Undo Segments
A basic structure of an Oracle Database is a Table. Oracle 10g supports

many types of Tables

5.Index Segment
Indexes are used to speed up the access to the tables. Oracle provides

many types of Indexes.These are:B*tree Indexes Bitmap Indexes ...

6.ORACLE BLOCKS
When a datafile is created and assigned to a tablespace, Oracle formats

the file into blocks for the actual storage. The size of each Oracle

block is determined by the DB_BLOCK_SIZE initialization parameter,

which is specified at the time of creation of database. Normally,

Oracle block size is in multiples of OS Block size, which is generally

512 bytes. An Oracle block is a basic unit of storage into which all

Oracle objects store data.

Oracle block is divided into three distinct segments:

Fixed Block Header
Variable Block Header, and
Data Storage Area

Structure of an Oracle Block

Fixed Block Header
Variable Block Header
ROW1 ROW2 ROW3 …
Row 1 Data
Row 2 Data
Row 3 Data
Free Space