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https://issues.apache.org/jira/browse/HIVE-24479?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Krisztian Kasa reassigned HIVE-24479:
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> Introduce setting to set lower bound of hash aggregation reduction.
> -------------------------------------------------------------------
>
> Key: HIVE-24479
> URL: https://issues.apache.org/jira/browse/HIVE-24479
> Project: Hive
> Issue Type: Improvement
> Components: Physical Optimizer
> Affects Versions: 4.0.0
> Reporter: Krisztian Kasa
> Assignee: Krisztian Kasa
> Priority: Major
> Fix For: 4.0.0
>
>
> * Default setting of hash group by min reduction % is 0.99.
> * During compilation, we check its effectiveness and adjust it accordingly in
> {{SetHashGroupByMinReduction}}:
> {code}
> float defaultMinReductionHashAggrFactor = desc.getMinReductionHashAggr();
> float minReductionHashAggrFactor = 1f - ((float) ndvProduct / numRows);
> if (minReductionHashAggrFactor < defaultMinReductionHashAggrFactor) {
> desc.setMinReductionHashAggr(minReductionHashAggrFactor);
> }
> {code}
> For certain queries, this computation turns out to be "0".
> This forces operator to skip HashAggregates completely and always ends up
> choosing streaming mode.
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