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Spatial databases (SDBs) are naturally distributed due to the data collection process, the distances between the geographical entities of interest, and the physical geographical boundaries. This calls for ways of interoperation among the distributed spatial databases. In order for the interoperability among distributed spatial databases to be possible, several issues have to be addressed. One of these is efficient distributed query processing and optimization for user queries that involve more than one SDB. A distributed query is any data manipulation statement that references databases at sites other than the query site; the site that initiated the query request. The processing sites are the sites where the actual processing of the query takes place, e.g., a join site is where one of the joins in the query takes place. In this paper, we focus on the efficient processing and optimization of complex spatial queries that involve combinations of spatial selections and joins. We have discussed strategies for efficiently processing a mixed query in which filter and refinement steps are involved.