A method and apparatus for determining distinct cardinality of a data sample using dual hash bitmaps. Two different bitmaps determine the distinct cardinality of the data sample (e.g., a column of data within a data table of a database). A small sized bitmap, M*sqrt(C*K), is used where...http://www.google.de/patents/US5802521?utm_source=gb-gplus-sharePatent US5802521 - Method and apparatus for determining distinct cardinality dual hash bitmaps 
