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The Oracle database server automatically modifies the values in the index when the
values in the corresponding columns are modified. Because the index contains less
data than the complete row in the table and because indexes are stored in a special
structure that makes them faster to read, it takes fewer I/O operations to retrieve the
data in them. Selecting rows based on an index value can be faster than selecting
rows based on values in the table rows. In addition, most indexes are stored in sorted
order (either ascending or descending, depending on the declaration made when you
created the index). Because of this storage scheme, selecting rows based on a range of
values or returning rows in sorted order is much faster when the range or sort order
is contained in the presorted indexes.
In addition to the data for an index, an index entry stores the ROWID for its associated
row. The ROWID is the fastest way to retrieve any row in a database, so the
subsequent retrieval of a database row is performed in the most optimal way.


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