public void visitChild(RelNode parent, RelNode child) {
   if (parent instanceof OLAPRel) {
     OLAPRel olapRel = (OLAPRel) parent;
     this.parentContext = olapRel.getContext();
   }
   OLAPRel olapChild = (OLAPRel) child;
   olapChild.implementRewrite(this);
 }
 @Override
 public EnumerableRel.Result visitChild(
     EnumerableRel parent, int ordinal, EnumerableRel child, EnumerableRel.Prefer prefer) {
   if (parent instanceof OLAPRel) {
     OLAPRel olapRel = (OLAPRel) parent;
     this.parentContext = olapRel.getContext();
   }
   return super.visitChild(parent, ordinal, child, prefer);
 }