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https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13546703#comment-13546703
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Phabricator commented on HIVE-3562:
-----------------------------------
njain has commented on the revision "HIVE-3562 [jira] Some limit can be pushed
down to map stage".
I thought about it, even with group bys, my question is still valid.
I think, there is a bug.
Do you think it would be simpler to allocate a heap with (upto) topN entries
instead - throw
the memory threshold out. If limit < threshold, use this optimization,
otherwise just ignore this
optimization.
INLINE COMMENTS
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:441 Isn't
there a bug here ?
You are using keyValues last entry to figure out whether it needs to be
expanded or not.
It may have an issue at the boundary - say entry 40th when a legit. entry is
found.
It might be simpler to pass the fact whether the entry was found or not.
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:468 This
is not true if an entry is being inserted in between.
I mean, if topN is 100, and we already have 100 entries.
If we are inserting 50th entry, we should not be increasing usage
REVISION DETAIL
https://reviews.facebook.net/D5967
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, HIVE-3562.D5967.2.patch,
> HIVE-3562.D5967.3.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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