Full Table scan & Index scan

It is a common misconception that all SQL queries on all tables in Oracle databases should be index driven.

In fact, using full-table scans can improve performance in two scenarios:

when querying very small tables and

when querying very large tables.

The exception to this of course is when the table has been created as an index-only table (available since Oracle 8) which means that the whole table is stored in a B-tree structure (although you may have pointers to overflow areas).

Source: http://www.smart-soft.co.uk/Oracle/oracle-performance-tuning-part1_1.htm