Indexes are created in a database to quickly locate relevant information. When properly used, indexes will speed SQL execution while reducing disk I/O and memory access. To better understand what an index is, think of the index of words at the back of any book. If you want to quickly locate information, you would refer to the index of words and navigate to that page.
A rowid is a pseudo-column that uniquely identifies a row. Each rowid contains the following information:
Object number of the object that the row belongs to
Data block of the datafile
Position of the row in the data block
Datafile number (it resides in)
This information helps an Oracle database to uniquely identify a record.
Indexes in Oracle are schema objects that are stored separately. Each index contains specified values from the indexed column along with the ROWID values for the rows that match them.
When accessing a small percentage of the rows of a large table, you would want to use an index....