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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