Hi, It turns out Calcite's nodes can change semantics without notification for the end-users.
Here are the release notes for Calcite 1.21, and it says **nothing** like "Ensure you handle Filter#variablesSet in case you implement Filter or in case you transform LogicalFilter in your rules" https://calcite.apache.org/news/2019/09/11/release-1.21.0/ On top of that, the in-core adapters fail to handle that properly. For example, MongoFilter discards Filter#variablesSet. Can we please stop changing the semantics of RelNodes or can we have a better way to detect the changes in the client code? What if we add a "version" property to the corresponding RelNodes, and we increment it every time the semantic changes? Then client APIs could be coded like "ok, I'm prepared to handle Project v4, and Filter v5" (e.g. make "version" required when registering a rule), and there will be a runtime error in case Calcite generates Filter v6 in runtime. --- Sample case: CALCITE-3183 Trimming method for Filter rel uses wrong traitSet CALCITE-4913 Correlated variables in a select list are not deduplicated The case there is that "correlation variables" are added to the Logical* nodes (e.g. LogicalFilter, LogicalProject). Unfortunately, that change is hard to notice: there's no compilation failure, and there's no runtime error. The old client code just discards "correlation variables", and I guess it would result in wrong results or something like that. Silent wrong results is a really sad outcome from the database. CALCITE-4913 / PR#2623 adds Project#variablesSet property as well, and I guess it would in the same hidden semantic loss. Vladimir
