GenMRUnionCtx uCtxTask = ctx.getUnionTask(union);
Task<? extends Serializable> uTask = null;
pos = UnionProcFactory.getPositionParent(union, stack);
Operator<? extends Serializable> parent = union.getParentOperators().get(pos);
mapredWork uPlan = null;
// union is encountered for the first time
if (uCtxTask == null) {
uCtxTask = new GenMRUnionCtx();
uPlan = GenMapRedUtils.getMapRedWork();