Multi-way spatial join selectivity for the ring join graph

Efficient spatial query processing is very important since the applications of the spatial DBMS (e.g. GIS, CAD/CAM, LBS) handle massive amount of data and consume much time. Many spatial queries contain the multi-way spatial join due to the fact that they compute the relationships (e.g. intersect) among the spatial data. Thus, accurate estimation of the spatial join selectivity is essential to generate an efficient spatial query execution plan that takes advantages of spatial access methods efficiently. For the multi-way spatial joins, the selectivity estimation formulae only for the two kinds of query types, tree and clique, have been developed. However, the selectivity estimation for the general query graph which contains cycles has not been developed yet. To fill this gap, we devise a formula for the multi-way spatial ring join selectivity. This is an indispensable step to compute the selectivity of the general multi-way spatial join whose join graph contains cycles. Our experiment shows that the estimated sizes of query results using our formula are close to the sizes of actual query results.