“…Due to the exploratory nature of data and to time restrictions, a user may prefer a fast but approximate answer to an exact but slow answer. Methods to deal with these issues consist of applying synopsis techniques, such as histograms [12,16,19,26], sketches [8] and wavelets [7,15]. Histograms are one of the techniques used in data stream management systems to speed up range queries and selectivity estimation (the proportion of tuples that satisfy a query), two illustrative examples where fast but approximate answers are more useful than slow and exact ones.…”