Andrey Mashenkov created IGNITE-21954:
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Summary: Reuse Calcite map-reduce aggregates code.
Key: IGNITE-21954
URL: https://issues.apache.org/jira/browse/IGNITE-21954
Project: Ignite
Issue Type: Improvement
Reporter: Andrey Mashenkov
*Motivation*
As for now we have custom code that splits aggregate function into map-reduce
stages. See class/method usage for details:
{code:java}
MapReduceAggregates.buildAggregates{code}
Some functions (e.g. AVG) are split into 2 functions which may have
incompatible types (e.g. COUNT returns BIGINT on map-phase, but SUM on
reduce-phase requires DECIMAL).
This forces adding a Projection in between, but this additional stage looks
synthetic and rises costs.
I've found Calcite already have `AggregateReduceFunctionsRule`, which splits
functions in different way (using `CASE-WHEN` clause instead of Project), and
can be reused.
*Suggestion*
Let's try to reuse Calcite rule and/or splitting functions, and drop
duplicating code from our codebase.
Otherwise, if there are obstacles, go the similar way at least.
Fix planner tests, which expects a Project between map and reduce phases.
*Expectation*
After this fix, we should observe no projection in between map and reduce
aggregate phases.
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