Oracle architecture-Introduction to databases, tablespaces, and instances

I. Databases

A database is a collection of data, while Oracle is a software system that manages these data sets. It is an object-relational database management system.

 Ii. tablespace

Tablespace is the logical ing between Oracle and the data in the physical database.. A database is logically divided into several tablespaces, each of which contains a set of logically associated structures. Each database has at least one tablespace (called the system tablespace ).

Each tablespace consists of one or more files on the same disk. These files are called data files ). A data file can belong to only one tablespace.

Shows the relationship between databases, tablespaces, and data files (images and some texts are imported from Oracle9i concepts Manual).

The figure above shows the following points:
· Each database is logically divided into one or more tablespaces.
· Each tablespace consists of one or more data files.
· The storage size of each tablespace is equal to the total size of all its data files.
· The storage size of each database is equal to the total size of all its tablespaces

Iii. Instances

Each running database corresponds to an oracle instance. When the Oracle database on the server is started, Oracle first allocates an area in the memory-called system global area (SGA), and then starts one or more Oracle processes.

Together with these processes, SGA is called an oracle instance.. The memory and Process Manager in the instance are used to efficiently access database data and provide services for one or more users.

A database can be opened by multiple instances, but only one database can be opened by one instance at any time. Multiple instances can run on the same machine at the same time, and they access each other's independent physical databases.

The database instance is as follows: (images and some texts are introduced from Oracle9i concepts manual)

After the instance is started, Oracle associates the instance with its corresponding physical database. This process is called "mounting). At this time, the database is ready to be opened. After the database is opened, only the administrator can close it. Normal users do not have the right to close the database.

(Note: You can operate the database instead of loading the database. You need to perform this step. How to open the databaseArticleDescription .)