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https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13511442#comment-13511442
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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".
As per jira comments, can you add a new parameter for the limit on k ?
INLINE COMMENTS
ql/src/java/org/apache/hadoop/hive/ql/optimizer/LimitPushdownOptimizer.java:73
Instead of checking for instanceof, can you add a method in Operator() -
something like
canLimitBePushed() with a lot of comments, and then the above operators can
have this to true.
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:358 Can
you add a heap based implementation as suggested in jira ?
ql/src/java/org/apache/hadoop/hive/ql/exec/ReduceSinkOperator.java:86 Can you
add lot of comments here - when is this set ?
what queries will it benefit etc. ?
common/src/java/org/apache/hadoop/hive/conf/HiveConf.java:460 add these in
hive-default.xml.template
ql/src/test/queries/clientpositive/limit_pushdown.q:7 can you add another
positive/negative test ?
explain select value, sum(key) from src group by value limit 10;
The limit should be used in the 2nd MR job if u r inserting into a table.
ql/src/test/queries/clientpositive/limit_pushdown.q:10 For the 2 negative
queries, can you insert into a table also -
then the optm. should help
REVISION DETAIL
https://reviews.facebook.net/D5967
To: JIRA, 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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