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https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13511477#comment-13511477
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Phabricator commented on HIVE-3562:
-----------------------------------
tarball has requested changes to the revision "HIVE-3562 [jira] Some limit can
be pushed down to map stage".
Not sure if I'm following the right protocol here. Marking this as "Request
Changes" per my comments.
INLINE COMMENTS
common/src/java/org/apache/hadoop/hive/conf/HiveConf.java:460 Minor
suggestion, feel free to ignore :) Prefer parameter name
"hive.limit.twophase.enable".
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:129 Why
not limit 0 (unless hive optimizes those away)?
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:305 Is
this variable not used at all?
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:323 This
code seems error prone. It has a side effect that's difficult to understand -
it is not obvious that the caller must wipe out value before the next row is
processed. Any particular reason we're caching it at all?
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:346 If o1
and o2 are null, shouldn't this return 0?
REVISION DETAIL
https://reviews.facebook.net/D5967
BRANCH
DPAL-1910
To: JIRA, tarball, navis
Cc: njain
> Some limit can be pushed down to map stage
> ------------------------------------------
>
> Key: HIVE-3562
> URL: https://issues.apache.org/jira/browse/HIVE-3562
> Project: Hive
> Issue Type: Bug
> Reporter: Navis
> Assignee: Navis
> Priority: Trivial
> Attachments: HIVE-3562.D5967.1.patch
>
>
> Queries with limit clause (with reasonable number), for example
> {noformat}
> select * from src order by key limit 10;
> {noformat}
> makes operator tree,
> TS-SEL-RS-EXT-LIMIT-FS
> But LIMIT can be partially calculated in RS, reducing size of shuffling.
> TS-SEL-RS(TOP-N)-EXT-LIMIT-FS
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