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Oracle9i External Tables

Before Oracle9i, the most common methods of manually performing complex transformations were:

• The extracted data would be loaded into staging tables in the data warehouse. The staged data would be transformed in the database and then used as input to programs that updated the permanent data warehouse tables.
   

• The data would be transformed in flat files stored outside of the database. When the transformation process was complete, the data would be loaded into the data warehouse.

Oracle9i introduces external tables, which provide a mechanism to view data stored in external sources as if it were a table in the database. This ability to read external data provides a more straightforward method of loading and transforming data from external sources. Administrators no longer need to reserve space inside the database for staging tables or write external programs to transform the data outside of the database environment. By making it no longer necessary to stage data in the Oracle database, Oracle9i’s external tables have essentially streamlined the ETL function by merging the transformation and loading processes.

External tables in Oracle are read-only and cannot have indexes built upon them. Their main use is a data source for more traditional Oracle table structures. Data warehouse administrators are able to use the CREATE TABLE AS SELECT…. and the INSERT INTO…..AS SELECT statements to populate Oracle tables using the external source as input.
 

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