WHAT IS ORACLE DATABASE?

The database has logical structures and physical structures. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting the access to logical storage structures.

PHYSICAL STRUCTURE OF DATABASE

The physical structure of database consists datafiles, controlfiles and redo log files

DATAFILES

Every Oracle database has one or more physical datafiles. The datafiles contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database

CONTROLFILE

Every time an instance of an Oracle database is started, its control file identifies the database and redo log files that must be opened for database operation to proceed. If the physical makeup of the database is altered (for example, if a new datafile or redo log file is created), then the control file is automatically modified by Oracle to reflect the change. A control file is also used in database recovery.

REDO LOGFILE

The primary function of the redo log is to record all changes made to data. If a failure prevents modified data from being permanently written to the datafiles, then the changes can be obtained from the redo log, so work is never lost.

ARCHIVE LOG FILES

The archive log files are mainly offline copy of redo logfiles since the redo log files are overwritten .Hence the data needed to recover from media failure can obtained from archive log files and data needed to recover from instance failure can be obtained by redo log files