A data base management system estimates the number of occurrences of values of query search keys in a data set by defining at least two independent hashing functions that map the values of the data set to buckets of respective hashing tables and maintaining a bucket count as each value from the data...http://www.google.de/patents/US5542089?utm_source=gb-gplus-sharePatent US5542089 - Method and apparatus for estimating the number of occurrences of frequent values in a data set
