Currently we provide answer to metadata query through RelMetadataProvider [1], there are some sub-classes of it:
RelMetadataProvider | |- VolcanoRelMetadataProvider |- ChainedRelMetadataProvider/DefaultRelMetadataProvider |- HepRelMetadataProvider |- CachingRelMetadataProvider |- ReflectiveRelMetadataProvider |- JaninoRelMetadataProvider The RelMetadataProvider has two methods: #apply and #handlers, the #apply method seems a programming interface and there is a demo code how we can use it: RelMetadataProvider provider; LogicalFilter filter; RexNode predicate; Function<RelNode, Metadata> function = provider.apply(LogicalFilter.class, Selectivity.class}; Selectivity selectivity = function.apply(filter); Double d = selectivity.selectivity(predicate); But let's see our RelOptCluster's member variables[2], there are MetadataFactory and RelMetadataQuery which all can be used to query the metadata, for MetadataFactory, there is a default impl named MetadataFactoryImpl which will invoke RelMetadataProvider#apply internally, for RelMetadataQuery, it will invoke RelMetadataProvider#handlers (finally composed and codeden by JaninoRelMetadataProvider). In our planning phrase, we can invoke RelOptRuleCall#getMetadataQuery to get the MQ and query the metadata. For extension of metadata handlers, we can set our customized RelMetadataProvider in RelOptCluster[3]. But for RelMetadataQuery, we have no way to extend it now, because the RelOptCluster always has a singleton instance [4] which is only the default implementation. My question is as follows: 1. Why we have 2 methods in RelMetadataProvider, and why we need the MetadataFactory and RelMetadataProvider#apply ? It seems that it's function is already been overriden by RelMetadataQuery(The difference is that MetadataFactory use Reflection and RelMetadataQuery use gened bytes code). 2. We should make the RelMetadataQuery in RelOptCluster pluggable. [1] https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/rel/metadata/RelMetadataProvider.java#L38 [2] https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/plan/RelOptCluster.java#L49 [3] https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/plan/RelOptCluster.java#L135 [4] https://github.com/apache/calcite/blob/b0e83c469ff57257c1ea621ff943ca76f626a9b7/core/src/main/java/org/apache/calcite/plan/RelOptCluster.java#L151 Best, Danny Chan
