Distributed Database

A distributed database is a network of database managed by multiple database servers that appears to a user as a single logical database. The data of all database in this network can be simultaneously accessed and modified. The prominent feature to be noted is that with the help of distributed database, the data of physically separate database can be logical combined and accessed by all users on a network. Each computer that manages a database in the distributed database is called as a node. The database to which the user is directly connected is called the local database and other database accessed by the user are referred to called as remote database.

Oracle provides utilities to back up -an Oracle database and also to move data between Oracle database. It also enables to load data into tables from other operating system files. This can be accomplished by using the below mentioned Utilities.

Export
Import
SQL"Loader

With Export, we can write data from an Oracle database into operating system files, whereas Import reads data from these back into an Oracle database. With the help of this combined Utility we can perform several tasks. Some of them are enumerated below.

Store Oracle data in Operating System files independent of any database
Store definitions of database objects like tables, clusters or indexes with or without data.
Store temporary data.
Back up only those tables whose data has being modified since the last export.
Restores tables which were dropped accidentally provided they were exported recently.

Export

Export writes data from an Oracle database into Operating systems file. These files can be read into another Oracle database using the import Utility (dealt in detail in latter part of this session). Before running Export there should be enough storage space on disk or tape to write Export file. Export file can be read only be Import. Therefore we cannot use them to transfer data to Non Oracle systems.

Export Modes

The objects exported are based on three modes namely Table, User and Full database modes. In the table mode we can export specific tables, rather exporting all of them. Export in table mode does not include cluster definitions. Therefore data is imported into unclustered table. By default it exports all the tables belonging to the user. User mode allows us to export all objects in our schema comprising tables, data, grants, indexes and so on. Thus it can be used to back up one database user. in Full database mode we can Export all database objects except those owned by SYS. Only DBA's can Export in this mode.

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