Github user julianhyde commented on the issue:
https://github.com/apache/drill/pull/671
Yes, CachingRelMetadataProvider is meant for this.
After https://issues.apache.org/jira/browse/CALCITE-604, providers are
considerably more efficient (not called via reflection), and RelMetadataQuery
contains a per-request cache (because some metadata does change, but slowly).
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