In the ddl backpressure graph, we want to compute the backpressure between relations.
So we need to know which are the fragments which are connected to external relations.
These fragments form the vertices of the graph.
We can get collection of backpressure values, keyed by vertex_id-vertex_id.
This function will return a map of fragment vertex id to relation id.
We can convert vertex_id-vertex_id
to relation_id-relation_id
using that.
Finally, we have a map of relation_id-relation_id
to backpressure values.