@Override public Void visitSubQuery(RexSubQuery subQuery) {
          subQuery.rel.accept(CorelMapBuilder.this);
          return super.visitSubQuery(subQuery);
        }
Creates a CorelMap by iterating over a {@link RelNode} tree.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveRelDecorrelator.java