What is an index? (some of the following was taken from Oracle’s Database Concepts, their links have been kept for easy access).
An index is an optional structure, associated with a table or table cluster, that can sometimes speed data access. By creating an index on one or more columns of a table, you gain the ability in some cases to retrieve a small set of randomly distributed rows from the table.
If a heap-organized table has no indexes, then the database must perform a full table scan to find a value. For example, without an index, a query of finding all students with a grade of 50 out of 100 requires the database to search every row in every table block for this value. This approach does not scale well as data volumes increase.
An index allows for faster retrieval of records. An index creates an entry for each value that appears in the indexed columns. It’s possible to create, rename and drop indexes. An index can be dropped, renamed or created without physically affecting the table for the index.
More information will follow soon.
Gambaru 