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From "Phabricator (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-3562) Some limit can be pushed down to map stage
Date Thu, 06 Dec 2012 15:09:10 GMT

    [ https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13511442#comment-13511442
] 

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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