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Instance, database, data dictionary and database creation relationship

2021-08-02

< pre > instances, databases, data dictionaries, and relational databases created by databases. Instances and databases are different from and related to each other. The data dictionary describes the logical and physical storage structure of the whole database. Database creation is actually to create a minimal physical storage structure for storing data dictionaries. The instance parameter file defines how to construct the memory structure and size, and the behavior of the background process. The unmount status indicates that the instance already exists in memory, but is not connected to the database on disk. DB in instance parameter file_ The name parameter indicates the database to connect to. control_ The control file defined by files defines the connection between the instance and the database. When the instance reads the control file, if the database name does not match, the database will not be mounted. In Mount mode, the instance is successfully connected to the control file. However, the control file does not exist or the database is damaged. It is impossible to mount. After the database is mounted, the instance can open the database by reading the control file information, locating and opening other files. In an open database instance, all available online redo log files and data files are opened. There is a mapping relationship between the data file and the tablespace in the control file. Enable the database to identify the data file of the system tablespace, so as to find the data dictionary. The data dictionary lets the instance resolve references to objects referred to in SQL code to the segments in which they reside, and work out where, physically, The objects are. Steps of database creation: ■ create the instance. ■ create the database and the data dictionary objects. ■ create the data dictionary views. The data dictionary is used to manage database data, but it is not understood by users. The view based on a data dictionary provides a method for users to understand. The script for creating and storing the data dictionary in the database is located in Oracle_ Home / RDBMS / Admin. The script is to create a database. The first script called is sql.bsq. This script will also call other scripts to create tables and other objects that make up the database. The database view creation script exists in Oracle_ Home / RDBMS / Admin, prefixed with cat, some of which will be executed during the database run to provide some features.

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