Histograms have been extensively studied and applied in the context of Selectivity Estimation (Kooi, 1980; Muralikrishna & DeWitt, 1998; Piatetsky-Shapiro et al., 1984; Poosala et al., 1996; Poosala, 1997), and are effectively implemented in commercial systems (e.g., Oracle Database, IBM DB2 Universal Database, Microsoft SQL Server) to query optimization purposes. In statistical databases (Malvestuto, 1993; Shoshani, 1997), histograms represent a method for approximating probability distributions. They have also been used in data mining activities, intrusion detection systems, scientific databases, that is, in all those applications which (i) operate on huge numbers of detailed records, (ii) extract useful knowledge only from condensed information consisting of summary data, (iii) but are not usually concerned with detailed information. Indeed, histograms can reach a surprising efficiency and effectiveness in approximating the actual distributions of data starting from summarized information. This has led the research community to investigate the use of histograms in the fields of database management systems (Acharya et al., 1999; Bruno et al., 2001; Gunopulos et al., 2000; Ioannidis & Poosala, 1999; Kooi, 1980; Muralikrishna & DeWitt, 1998; Piatetsky-Shapiro et al., 1984; Poosala, 1997; Poosala & Ioannidis, 1997), online analytical processing (OLAP) systems (Buccafurri et al., 2003; Cuzzocrea, 2005a; Cuzzocrea & Wang, 2007; Furfaro et al., 2005; Poosala & Ganti, 1999), and data stream management systems (Guha et al., 2001; Guha et al., 2002; Thaper et al., 2002), where, specifically, compressing data is mandatory in order to obtain fast answers and manage the endless arrival of new information, as no bound can be given to the amount of information which can be received.