Marcin Gorawski Rafal Malczok Aggregation and Analysis of Spatial Data by Means of Materialized Aggregation Tree. 24-33 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3261&spage=24 2004 conf/advis/2004 ADVIS db/conf/advis/advis2004.html#GorawskiM04