Question about ReferenceRel

[eedalong]

Hi guys, I am really confused about this ReferenceRel.

1. It is defined in AggregateFunction?
   

2. How we suppose to decide the subtree_ordinal for one operator to make it referencable by other Rels?

[westonpace]

The ReferenceRel message, as it is currently implemented, is incomplete and ambiguous. There was an attempt to clarify things in #288 but that did not merge. However, there is still some good discussion and I recommend you check it out.

Someone should either finish what #288 started or they should remove ReferenceRel. I believe this has been low priority since no one is using it or has needed it yet.

[westonpace]

I will attempt to answer your questions:

It is defined in AggregateFunction?

Yes, that is wrong. It should be a top-level message in algebra.proto.

How we suppose to decide the subtree_ordinal for one operator to make it referencable by other Rels?

The subtree_ordinal here is an index into Plan::relations:

message Plan {
  ...
  // one or more relation trees that are associated with this plan.
  repeated PlanRel relations = 3;

So if the subtree_ordinal is 2 then it is the third item in Plan::relations.

[jacques-n]

ReferenceRel is now complete and available for use.