Abstract: A technique is provided for incrementally maintaining column cardinality estimates in database management systems. The system catalog table containing a cardinality estimate for a column is extended to include an appropriate data structure. A modified linear counting technique is used in a first embodiment of a method for column cardinality estimation. Moreover, a modified logarithmic counting technique is used in a second, preferred embodiment of a column cardinality estimation method to reduce storage requirements for the data structure. The cardinality estimate is produced by an initial scan of the data but is then further maintained without requiring a full scan of the data. Data changes are reflected incrementally in modifications to the initial cardinality estimate, keeping the cardinality statistics more current with respect to the database condition.

Abstract: A system, method and computer readable medium for estimating a column cardinality value for a column in a partitioned table stored in a plurality of nodes in a relational database is disclosed. According to one embodiment of the present invention, a plurality of column values for the partitioned table stored in each node are hashed, and a hash data set for each node is generated. Each of the hash data sets from each node is transferred to a coordinator node designated from the plurality of nodes. The hash data sets are merged into a merged data set, and an estimated column cardinality value for the table is calculated from the merged data set.