[jira] [Created] (DRILL-6836) Eliminate StreamingAggr for COUNT DISTINCT

["Boaz Ben-Zvi (JIRA)" <ji...@apache.org>]

Boaz Ben-Zvi created DRILL-6836:
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             Summary: 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
             Fix For: 1.16.0


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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