Navis created HIVE-4002:
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Summary: Fetch task aggregation for simple group by query
Key: HIVE-4002
URL: https://issues.apache.org/jira/browse/HIVE-4002
Project: Hive
Issue Type: Improvement
Components: Query Processor
Reporter: Navis
Assignee: Navis
Priority: Minor
Aggregation queries with no group-by clause (for example, select count(*) from
src) executes final aggregation in single reduce task. But it's too small even
for single reducer because the most of UDAF generates just single row for map
aggregation. If final fetch task can aggregate outputs from map tasks,
shuffling time can be removed.
This optimization transforms operator tree something like,
TS-FIL-SEL-GBY1-RS-GBY2-SEL-FS + FETCH-TASK
into
TS-FIL-SEL-GBY1-FS + FETCH-TASK(GBY2-SEL-LS)
With the patch, time taken for auto_join_filters.q test reduced to 6 min (10
min, before).
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