1. A method for query processing in a massively parallel processing (MPP) database system, the method comprising: receiving, at a coordinator process, a join query associated with a plurality of tables of the MPP database system; generating, at the coordinator process that runs on the MPP, an execution plan tree for the join query; processing, at each of a plurality of processing nodes of the MPP, the execution plan tree to obtain join query results; upon detecting a next join operator below a top join operator in the execution plan tree at each of the processing nodes, forwarding to the other processing nodes a sub-tree for the next join operator; and receiving, at each of the processing nodes from the other processing nodes, sub-tree processing results.