Method and apparatus for estimating the number of occurrences of frequent values in a data set

摘要

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 set is mapped to the hashing tables. A bucket is defined to be a "popular" bucket if the bucket count of the value exceeds a predetermined threshold. If all of the buckets to which a value is mapped are designated popular buckets, that value is designated an "active" value. Once a value is designated active, statistical data related to the value is collected. Estimates of the most frequently occurring values in the data set are generated from the collected statistical data. In this way, a data base management system can more effectively produce a search plan that provides an efficient response to user queries.