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https://issues.apache.org/jira/browse/DRILL-6836?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Boaz Ben-Zvi updated DRILL-6836:
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Fix Version/s: (was: 1.16.0)
1.17.0
> Eliminate StreamingAggr for COUNT DISTINCT
> ------------------------------------------
>
> Key: DRILL-6836
> URL: https://issues.apache.org/jira/browse/DRILL-6836
> Project: Apache Drill
> Issue Type: Improvement
> Components: Execution - Relational Operators, Query Planning &
> Optimization
> Affects Versions: 1.14.0
> Reporter: Boaz Ben-Zvi
> Assignee: Boaz Ben-Zvi
> Priority: Minor
> Fix For: 1.17.0
>
>
> The COUNT DISTINCT operation is often implemented with a Hash-Aggr operator
> for the DISTINCT, and a Streaming-Aggr above to perform the COUNT. That
> Streaming-Aggr does the counting like any aggregation, counting each value,
> batch after batch.
> While very efficient, that counting work is basically not needed, as the
> Hash-Aggr knows the number of distinct values (in the in-memory partitions).
> Hence _a possible small performance improvement_ - eliminate the
> Streaming-Aggr operator, and notify the Hash-Aggr to return a COUNT (these
> are Planner changes). The Hash-Aggr operator would need to generate the
> single Float8 column output schema, and output that batch with a single
> value, just like the Streaming -Aggr did (likely without generating code).
> In case of a spill, the Hash-Aggr still needs to read and process those
> partitions, to get the exact distinct number.
> The expected improvement is the elimination of the batch by batch output
> from the Hash-Aggr, and the batch by batch, row by row processing of the
> Streaming-Aggr.
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