Make RelBuilder configurable, so that particular optimizations can easily be turned off.
I propose to add a class RelBuilder.Config, which is immutable and has a public final field for each configuration property; also a class RelBuilder.ConfigBuilder to create a config.
RelBuilder.create(FrameworkConfig frameworkConfig) will get a config by calling frameworkConfig.getContext().unwrap(RelBuilder.Config.class).
Going forward, any new features that add "optimizations" to RelBuilder would need to have a corresponding flag in Config to switch them off. A feature would not be considered "complete" if it did not have tests and a switch.