Overview on Oracle External tables
External tables can read flat files (that follow some rules) as though they were ordinary (although read-only) Oracle tables. Therefore, it is convenient to use external tables to load flat files into the DB. External can be used like SQL tables in querying purpose. So we can use it for insert and update of data into permanent tables. No DML can be performed on external tables but they can be used for query, join and sort operations. Views and synonyms can be created against external tables. They are useful in the ETL process of data warehouses since the data doesn’t need to be staged and can be queried in parallel.