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Posted to dev@hive.apache.org by "Navis (JIRA)" <ji...@apache.org> on 2013/02/08 10:03:23 UTC

[jira] [Created] (HIVE-4002) Fetch task aggregation for simple group by query

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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