There is provided a method of using a bitmap filter to filter a data source as part of a join operation. A set bit at a bit location of the bitmap filter is associated with an entry of the data source that comprises an identifier that matches a filter parameter. A row identifier of a row in another data source that corresponds to the identifier is determined based on a sum of a number of bits set in bit locations that are between the bit location of the set bit associated with the entry of the data source and a bit location in the bitmap filter corresponding to a different, known, row identifier of a different row in the another data source. The row of the another data source corresponding to the determined row identifier is interrogated to obtain information defining the identifier that matches the filter parameter.