1. A method to process database queries, the method comprising: accessing, by executing an instruction with a processor, a plurality of bottom-k sketches summarizing a plurality of data sets in response to receiving a query, a first one of the bottom-k sketches including pairs of ranks and weights for a first number of items from a first one of the data sets, the first number of items having smallest ranks among a set of items included in the first one of the data sets, the first one of the bottom-k sketches also identifying a next largest rank corresponding to a smallest remaining rank among the remaining items included in the first one of the data sets but not included in the first one of the bottom-k sketches; determining, by executing an instruction with the processor, a combination sketch based on a combination of the plurality of bottom-k sketches, the combination sketch containing items from the plurality of bottom-k sketches having respective ranks less than a limiting rank, the limiting rank being a minimum of a plurality of next largest ranks corresponding respectively to the plurality of bottom-k sketches, the combination sketch including more items than are included in any one of the plurality of bottom-k sketches; and determining, by executing an instruction with the processor, a query result based on the combination sketch, the query result being responsive to the query.