explicitly insert logical exchanges where we might require data shuffling; and
then determine whether those exchanges are real data shuffle or possible passthrough;
then we determine whether to assign more/less servers to run either side of the exchanges
Current solution
This PR (#11976) only addresses step-2: to give it a better idea on whether the RelDistribution is the same before & after an exchange. for this purpose, it is OK (at this point) to only keep one of the 2 sides for JOIN Rel.
Needs revisit
there are several problems
should we explicitly insert exchange or should we use other abstractions?
there are other ways to add exchange nodes that are more "Calcite-suggested" when managing the Exchange insert instead of applying them during optimization IMO.
should the exchange insert be RelDistribution-based or physical-based?
we are mixing the concept of Exchange usage: it can mean (1) altering logical RelDistribution, (2) indicating there potentially could be physical layout differences, (3) whether we can apply leaf-stage optimization
although (3) will be addressed by #11937, we should consider whether to still use ExchangeNode as our abstraction or create our own to avoid confusion
should we apply RelDistribution trait before or after Exchange insert
currently we have to do this after insert, but technically if we addresses question (2) we can potentially apply that beforehand
ultimately utilizing Exchange was a quick decision during early stage multi-stage engine development and it might not have been the best option. it is worth taking some time to revisit
This discussion stems from: #11976
Status quo
we currently
Current solution
This PR (#11976) only addresses step-2: to give it a better idea on whether the RelDistribution is the same before & after an exchange. for this purpose, it is OK (at this point) to only keep one of the 2 sides for JOIN Rel.
Needs revisit
there are several problems
ultimately utilizing Exchange was a quick decision during early stage multi-stage engine development and it might not have been the best option. it is worth taking some time to revisit