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https://issues.apache.org/jira/browse/HIVE-4002?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13751333#comment-13751333
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Phabricator commented on HIVE-4002:
-----------------------------------
yhuai has commented on the revision "HIVE-4002 [jira] Fetch task aggregation
for simple group by query".
INLINE COMMENTS
ql/src/java/org/apache/hadoop/hive/ql/exec/Operator.java:582 I did not mean
we cannot have a recursive flush method. I meant that Demux and Mux operators
should not use a recursive flush method.
REVISION DETAIL
https://reviews.facebook.net/D8739
To: JIRA, navis
Cc: yhuai
> 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
> Attachments: HIVE-4002.D8739.1.patch, HIVE-4002.D8739.2.patch,
> HIVE-4002.D8739.3.patch, HIVE-4002.D8739.4.patch
>
>
> 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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