This is the default behavior for CREATE INDEX. The Oracle Rdb
    optimizer collects approximate cardinality values for the index
    columns to help in future query optimization. Note that no extra
    I/O is incurred to collect these values and, therefore, adjacent
    key values from other index nodes can not be checked. Hence, some
    inaccuracy may be seen for these indexes. In most cases, this is
    adequate for query optimizations.