// create a walker which walks the tree in a DFS manner while maintaining
// the operator stack.
Map<Rule, NodeProcessor> opRules = new LinkedHashMap<Rule, NodeProcessor>();
opRules.put(new RuleRegExp(new String("Set parallelism - ReduceSink"),
ReduceSinkOperator.getOperatorName() + "%"),
new SetReducerParallelism());
opRules.put(new RuleRegExp(new String("Convert Join to Map-join"),
JoinOperator.getOperatorName() + "%"), new ConvertJoinMapJoin());
// The dispatcher fires the processor corresponding to the closest matching