Oracle database instances and related concepts ZZ

A Complete Oracle database consists of an Oracle database instance and a database.

1) A database is a collection of physical files (data files, control files, online logs, parameter files, etc );

2) an Oracle database instance is a group of Oracle background processes/threads and the shared memory zone allocated on the server.

When the Oracle database server is started, an oracle instance is actually created in the server's memory (that is, the shared memory is allocated in the server memory and the related background memory is created ),Then, the Oracle database instance accesses and controls the data files on the disk. Oracle has a large memory speed and becomes a global zone (SGA ).

I. Databases, tablespaces, and data files

1. Database

A database is a collection of data. Oracle is a database management system and a relational database management system.

Generally, the "Database" refers to not only physical data sets, but also physical data and database management systems. That is, the combination of physical data, memory, and operating system processes.

The database data is stored in the table. The relationship between data is defined by columns, that is, the field we usually talk about. Each column has a column name. Data is stored in a table in the form of rows (usually called records. Tables can be associated with each other. The above is a simple description of the relational model database.

Of course, Oracle also provides the most powerful support for object-oriented structured databases. objects can be related to other objects or contain other objects. For oo databases, we will discuss them in detail later. Generally, our discussions are based on Relational Models.

2. tablespace and files

Regardless of the relational structure or oo structure, Oracle databases store their data in files. The database structure provides logical ing of data files, allowing separate storage of different types of data. These logical partitions are called tablespaces.

A table space is the logical division of a database. Each database has at least one table space (called a system table space ). To facilitate management and improve operation efficiency, some additional table spaces can be used to divide users and applications.Program. For example, the user tablespace is used by general users, and the RBS tablespace is used by rollback segments. A tablespace can belong to only one database.

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. After oracle7.2, the data file size can be changed during creation. To create a new tablespace, you must create a new data file. Once a data file is added to a tablespace, it cannot be moved from the tablespace or be associated with other tablespaces.

If the database is stored in multiple tablespaces, You can physically separate their data files on different disks. In the method of planning and coordinating database I/O requests, the preceding data segmentation is an important method.

3. The storage structure of the Oracle database is divided into logical and physical storage structures:

1) logical Storage Structure: describes how Oracle organizes and manages data;

2) physical storage structure: it is used to describe how data is organized and managed outside the Oracle operating system.

Ii. Oracle database instances

To access data in the database, Oracle uses a group of background processes shared by all users. In addition, there are some storage structures (collectively referred to as system gloabl area, that is, SGA) used to store the data recently queried from the database. The data block cache area and the SQL shared SQL pool are the largest parts of SGA, which generally accounts for more than 95% of the SGA memory. By reducing the number of I/O operations on data files, these storage areas can improve database performance.

An Oracle database instance, also known as a server, is a collection of storage structures and background processes used to access database file sets. A database can be accessed by multiple instances (this is an Oracle Parallel Server option ).

The instance size and composition parameters are stored in the init. ora file (spfile in 9i ). This file must be read when the Oracle database instance is started and can be modified by the database administrator at runtime. Any modification to this file will only start at the next startup. The init. ora file of an instance usually contains the Instance name: if an instance is named orcl, The init. ora file is usually named initorcl. ora. Another configuration file config. ora is used to store variable values that will not be changed after the database is created (such as the database block size ). The config. ora file of the Instance usually contains the Instance name. If the Instance name is orcl, config. ora is usually named configorcl. ora. To facilitate the setting of the config. ora file, in the init. ora file of the instance, the file must be listed using the ifile parameter as the inclusion file.

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