Stamatis > Maybe we could say that if RelOptRule#getOutTrait returns null then input traits Stamatis >and output traits should always match. It doesn't sound bad to allow only a Stamatis > Converter to modify traits.
Unfortunately that does not solve the problem of selecting the proper rules to fire. We want to select rule before we run it and realize the convention does not match. In the mean time, https://github.com/apache/calcite/pull/1019 passes all the tests in Calcite. Note: I've made the approach a bit more generic, so it applies for other traits as well, not just Convention. Any thoughts? Vladimir
