Why is DruidQuery avoiding having inputs ?

[Muhammad Gelbana]

I see that *DruidQuery* intentionally avoids having inputs by not
overriding *AbstractRelNode.getInputs()* and asserting that no inputs are
providing while copying itself:

@Override
> public RelNode copy(RelTraitSet traitSet, List<RelNode> inputs) {
>     *assert inputs.isEmpty();*
>     return this;
> }
>

Would someone please explain the purpose of doing this ?

Thanks,
Gelbana