Disclosed herein is a computer-implemented method of processing a query using a columnstore comprising a plurality of segments, the method comprising: receiving a query comprising a parameter; determining a key corresponding to the parameter; identifying a mapping structure, from a plurality of mapping structures, relating to said key, wherein each mapping structure corresponds to a respective segment of the columnstore; interrogating the identified mapping structure to determine a value corresponding to the key, wherein the value identifies an entry of an index table of a plurality of index tables that corresponds to the respective segment, wherein the entry of the index table identifies a row of the respective segment of the columnstore relating to the parameter; and retrieving data relating to the parameter from a data source based on data stored in the entry of the index table.