Oracle Database Administration: The Essential Refe

Logical Database Structures

In addition to the physical files used by the database, Oracle maintains many types of logical structures. Some of the most important are summarized here:

Tablespaces

The basic storage allocation in an Oracle database is a tablespace. Each tablespace is composed of one or more physical (operating system) files. Every database is created with the SYSTEM tablespace. Other tablespaces are created by the DBA. Note, however, that if you allow the Oracle Installer to create a database automatically, it will create additional tablespaces (see Chapter 2).

Schemas

In Oracle, a schema is essentially the same as an account or a username. Each object in the database is owned by a schema. Every Oracle database is created with two initial schemas: SYS, which is used to store the data dictionary, and SYSTEM, which often stores some data dictionary extensions as well as critical tables for other tools. Other schemas are created by the DBA. Each schema can be granted quotas in any tablespace. There is no necessary relationship between a schema and a tablespace.

Segments

Each object that takes up space is created as one or more segments. Each segment can be in one and only one tablespace.

Extents

Each segment is composed of one or more extents. An extent is a contiguous allocation of space within a datafile of a tablespace. At the time a segment is created, you can specify the size of the initial and next extents, as well as the minimum and maximum number of extents.

Rollback ...