Logical Storage Structure of Oracle Database
An oracle server consists of both a database and an instance. In Oracle terminology, database refers to only the physical files on disk. These are the files that store the data itself, the database state information in the control files, and the changes made to the data in the redo log files. The term instance refers to the Oracle processes and memory structures that reside in the server’s memory and access an Oracle database in disk.
Database: The collection of all physical files on disk that is associated with a single Oracle instance.
Instance: The collection of memory structures and Oracle background processes that operates against an Oracle database.
It’s also important to differentiate between the logical and physical structures of the database. The logical structures represent components such as table -t you normally see from a user’s point of view. The physical structures are the underlying storage methods on disk – the physical files that compose the database.
Logical Storage Structures
The Oracle database is divided into increasingly smaller logical units to manage, store, and retrieve data efficiently and quickly.
Tablespace is a logical grouping of database objects, usually to facilitate security, performance, or the availability of database objects such as tables and indexes. A tablespace is composed of one or more datafiles on disk.
Tablespace is the highest level of logical objects in the database. A database consists of one or more tablespaces.
Segment is a set of extents allocated for a single type of object, such as a table. A tablespace is further broken down into segments. A database segment is a type of object that a user typically sees, such as a table.
Extent is a contiguous group of blocks allocated for use as part of a table, index, and so forth. The next lowest logical grouping in a database is the extent.
Blocks is the smallest unit if allocation in an Oracle database. One or more database blocks compose a database extent. At the other end of the spectrum of logical objects is the database block (also known as oracle block), the smallest unit of storage in an Oracle database. Every database block in a tablespace has the same number of bytes.