An index is a mechanism that the SQL query optimizer can use to make table access more efficient. Indexes enhance data access by providing a more-or-less direct path to stored data to avoid performing full table scans to locate the small number of rows you typically want to retrieve or update.
The database parallel architecture makes indexing an aid to better performance, not a crutch necessary to ensure adequate performance. Full table scans are not something to be feared in the database environment. This means that the sorts of unplanned, ad hoc queries that characterize the data warehouse process, and that often are not supported by indexes, perform very effectively for the database using full table scans.