1. A method for operating a database apparatus, comprising: providing a distributed memory apparatus; and storing compressed representations of data identifiers in the distributed memory apparatus as two-level indexed data structures, wherein a first level of each data structure includes a first unique set of decreasing integers, wherein a second level of the data structure includes a plurality of unique sets of decreasing integers, wherein each of the plurality of unique sets is linked to one of the integers in the first unique set, wherein each integer in the first unique set represents a first portion of the data identifier and each unique integer in the plurality of sets represent a second portion of the data identifier.