Understanding Database Structure:

An Oracle database is a set of data which is treated as a unit. The main purpose of a database is to retrieve and store associated information. The database has physical structures and logical structures.

Logical Database Structure

Logical Database Structures holds many divisions like as tablespace, data blocks, schema objects, extents and segments. The subsequent sections describe this.

Tablespaces

A database is separated into logical storage units known as tablespaces. A tablespace is used to group associate logical structures together. For instance, tablespaces generally group all of an application's objects to easy certain administrative operations.

Schemas and Schema Objects

A schema is a set of objects. The Schema objects are the logical structures which directly refer to the database's data. Schema objects involves such structures as tables, views, clusters,  stored procedures, sequences, indexes, synonyms and database links. (There is no relationship among  a  tablespace  and  a  schema;  objects  in  the  similar  schema  can  be  in  variant tablespaces and a tablespace can hold objects from variant schemas.)

Oracle Data Blocks

At the excellence level of granularity an Oracle database's data is stored in data blocks. One data block corresponds to an exact number of bytes of physical database space on the disk. The data block size is specified for each Oracle database when the database is created. A database allocates and uses free database space in Oracle data blocks.

Extents

The next level of logical database space is known as an extent. An extent is a specific number of contiguous data blocks and obtained in a single allocation and used to store a specific type of information.

Segments

The level of logical database storage above an extent is known as a segment. The segment is a set of extents allocated for a certain logical structure. For instance, the different categories of segments involve the following:

Data Segment

Each non-clustered table has a data segment. Whole of the table's data is stored in the extents of its data segment. Every cluster has a data segment. A data of every table in the cluster is stored in the cluster's data segment.

Index Segment

Every index has an index segment the index segment is stores all of its data.

Rollback Segment

One or more rollback segments are created through the database administrator for a database to temporarily store "undo" information.

Temporary Segment

Temporary segments are created through Oracle when a SQL statement required a temporary work area to complete execution. Whenever the statement finishes execution the temporary segment's extents are returned to the system for future use.