unload with union query fails with ERROR[4066]

[Julie Thai]

On cluster with build 20141126_0830:

SQL>unload with purgedata from target
into '/bulkload/QA_unload_tests/loc_queries'
select ps_partkey, ps_suppkey, ps_availqty, ps_supplycost, ps_comment
from g_tpch2x.partsupp where ps_partkey < 90000
UNION
select ps_partkey, ps_suppkey, ps_availqty, ps_supplycost, ps_comment
from g_tpch2x.partsupp where ps_partkey between 180000 and 200000;
*** ERROR[4066] The operands of a UNION must be of equal degree. [2014-12-03 12:26:00]

[khaled Bouaziz (khaled-bouaziz)]

the display tool showed that the unload node is a child of the union in this case and the second child is a select statement
        root
           |
         scalar_aggr
             \
         union
          / \
     root root
        | |
   unload scan

what we want is a plan where union is a child of the unload node in this case.
the issue seems to be in the parser production rules for t he unload command. after changing t he parser the issue does not sem to happen any more and the plan we get now is like this

        root
           |
         unload
             |
         group by
             |
         union
          / \
     scan scan

the change in the parser also allowed us to use query_expression with no additional parser conflicts