Hyunsik Choi created TAJO-943:
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Summary: Preaggregation can be disabled by optimization hint or
session variable
Key: TAJO-943
URL: https://issues.apache.org/jira/browse/TAJO-943
Project: Tajo
Issue Type: Improvement
Components: distributed query plan
Reporter: Hyunsik Choi
Priority: Minor
Aggregation operators requires two phases and one shuffle as follows:
* preaggregation -> hash shuffle -> aggregation
Preaggregation usually reduces the intermediate volume between two stages. It
usually is an effective way to speed up aggregation operation.
But, in some case where the number of distinct grouping keys are lager,
preaggregation may not reduce the intermediate data while consuming expensive
operation costs.
It would be useful if we disable preaggregation through some session variable
or optimization hint.
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