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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