Stamatis Zampetakis created CALCITE-5705:
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Summary: Generalize RemoveEmptySingleRule to work with arbitrary
relations and pruning configurations
Key: CALCITE-5705
URL: https://issues.apache.org/jira/browse/CALCITE-5705
Project: Calcite
Issue Type: Improvement
Components: core
Reporter: Stamatis Zampetakis
Assignee: Stamatis Zampetakis
Fix For: 1.35.0
Currently
[RemoveEmptySingleRule|https://github.com/apache/calcite/blob/b0b27e8872b33c5ab203e0e2365d267a594c80be/core/src/main/java/org/apache/calcite/rel/rules/PruneEmptyRules.java#LL285C23-L285C44]
can only transform {{SingleRel}} relations to empty. However, the logic inside
the {{matches}} method is at the most part capable of handling any kind of
relation including those that have multiple children.
By generalizing the {{RemoveEmptySingleRule}} to work with arbitrary relations
we can refactor other pruning rules such as those created by
[CorrelateLeftEmptyRuleConfig|https://github.com/apache/calcite/blob/b0b27e8872b33c5ab203e0e2365d267a594c80be/core/src/main/java/org/apache/calcite/rel/rules/PruneEmptyRules.java#L588]
without the need for creating more classes and duplicating code.
Moreover by changing the constructor to accept {{PruneEmptyRule.Config}}
instead of {{RemoveEmptySingleRuleConfig}} we can reduce code duplication
further since configurations such as {{ZeroMaxRowsRuleConfig}} and
{{SortFetchZeroRuleConfig}} could be modified to create instances of
{{RemoveEmptySingleRule}}.
This is mainly a refactoring to simplify pruning rules and remove duplicate
logic. The change is fully backwards compatible.
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