Specifies that sorted and hashed indexes are stored in a
    compressed form.

    If compression is enabled, Oracle Rdb uses run-length compression
    to compress a sequence of space characters (octets) from text
    data types and binary zeros from nontext data types. Different
    character sets have different representations of the space
    character. Oracle Rdb compresses the representation of the space
    character for the character sets of the columns comprising the
    index values.

    You cannot disable index compression using the ALTER INDEX
    statement once you specified the ENABLE COMPRESSION clause of
    the CREATE INDEX statement.

    For more information on compressed indexes, see the Oracle Rdb
    Guide to Database Design and Definition.