A large portion of the metadata api is functionally deprecated, we should make it explicit.
Apache Calcite currently has 2 ways deriving metadata about rel node, one through runtime generated handler classes that are compiled using Janino, and the other using java reflection. The former will be referred to as Janino and the latter as java reflection for brevity. Java reflection is not used due to performance reason in calcite main, nor is it tested, and using it can lead to degraded performance due to interactions with caching of the janino handlers. Both share a common way of registering classes with implementation by using ReflectiveRelMetadataProvider and ChainedRelMetadataProvider. There are also 2 different subsystems for supporting metadata caching, one where a reference to a cache is stored in the metadata query and the other where caching is a wrapper pattern around the providers. There are also 2 separate ways for invalidating the caching, one with a counter stored in the planner used by java reflection api, and another where volcano planner tracks the parents of all rel nodes that it mutates and then explicitly invalidates those parents used by the janino api.
So the following could be removed without the lose of any functionality:
- CachingRelMetadataProvider - used for java reflection base metadata caching
- RelOptPlanner.getRelMetadataTimestamp - used by java reflection to invalidate the metadata cache
- RelOptPlanner.registerMetadataProviders(List<RelMetadataProvider> list) - used by java reflection API to support of Hep and Volcano nodes.
- MetadataFactory - used to caching an internal artifact of the implementation of java reflection API
- HepRelMetadataProvider - used by java reflection to support Hep node delegation to a child node.
- VolcanoRelMetadataProvider - used by java reflection to support RelSubset node delegation to a child node.
- AbstractRelNode.metadata - i think was an even older api that now bridges to the java reflection api
- RelNode.metadata - see above
- MetadataDef.metadtaClass - used by java reflection and for a unique descriptive name of the janino generated handler. Janino dependency on it is easily broken.
- ReflectiveRelMetadataProvider. map and metadataClass0 - used by java reflection implementation
- RelMetadataProvider.apply() - most of the actual implementation java reflection API
Metadata and MetadataDef could also be removed, however removing it would require reworking portions of janino code do so.