[jira] [Created] (CALCITE-5314) Prune empty parts of a query by exploiting stats/metadata

["Stamatis Zampetakis (Jira)" <ji...@apache.org>]

Stamatis Zampetakis created CALCITE-5314:
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             Summary: Prune empty parts of a query by exploiting stats/metadata
                 Key: CALCITE-5314
                 URL: https://issues.apache.org/jira/browse/CALCITE-5314
             Project: Calcite
          Issue Type: Improvement
          Components: core
            Reporter: Stamatis Zampetakis


Currently there is a collection of rules (i.e., [PruneEmptyRules|https://github.com/apache/calcite/blob/4ef9ffe0d4afb80fe95e66d4d9c9e6f5939f70a8/core/src/main/java/org/apache/calcite/rel/rules/PruneEmptyRules.java]) which remove sections of a query plan when it does not produce any rows.

At the moment, the removal requires an empty {{Values}} operator to be in the plan in order to take effect. However, there are cases where queries involve empty relations/tables and in that case we could remove parts of the plan as well.

The information if a relation is empty or not can be derived from metadata. For example, we could check if the  [RelMdMaxRowCount|https://github.com/apache/calcite/blob/4ef9ffe0d4afb80fe95e66d4d9c9e6f5939f70a8/core/src/main/java/org/apache/calcite/rel/metadata/RelMdMaxRowCount.java] returns zero to derive that the relation is empty.

Implementation wise there are various alternatives to consider:
 * modify the existing PruneEmptyRule to consider metadata;
 * add new pruning rules relying exclusively on metadata;
 * add new rules checking metadata and turning the empty relation to an empty {{Values}} which can then be handled by existing rules




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