A database can be considered from both a physical and logical perspective.
Physical data is data viewable at the operating system level. For example, operating system utilities such as the Linux ls and ps can list database files and processes. Logical data such as a table is meaningful only for the database. A SQL statement can list the tables in an Oracle database, but an operating system utility cannot.
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The database has physical structures and logical structures. Because the physical and logical structures are separate, you can manage the physical storage of data without affecting access to logical storage structures. For example, renaming a physical database file does not rename the tables whose data is stored in this file.
Physical Storage Structures
The physical database structures are the files that store the data.
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When you execute a CREATE DATABASE statement, the following files are created:
- Data filesEvery Oracle database has one or more physical data files, which contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the data files.
- Control filesEvery Oracle database has a control file. A control file contains metadata specifying the physical structure of the database, including the database name and the names and locations of the database files.
- Online redo log filesEvery Oracle Database has an online redo log, which is a set of two or more online redo log files. An online redo log is made up of redo entries (also called redo log records), which record all changes made to data.
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Many other files are important for the functioning of an Oracle database server. These include parameter files and networking files. Backup files and archived redo log files are offline files important for backup and recovery.
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