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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-6089) Validate That Planner Does Not Assume HashJoin Preserves Ordering for FS, MaprDB, or Hive
Date Thu, 08 Feb 2018 19:13:00 GMT

    [ https://issues.apache.org/jira/browse/DRILL-6089?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16357433#comment-16357433
] 

ASF GitHub Bot commented on DRILL-6089:
---------------------------------------

Github user HanumathRao commented on the issue:

    https://github.com/apache/drill/pull/1117
  
    @ilooner Changes look fine to me. However, as discussed offline I couldn't reproduce the
plan which was sorting one of the inputs for a HASHJOIN. This might be because in the hashjoinprule
we are not asking for those traits in the first place. However, I do agree that this fix can
be a safegaurd in case if that plan happens to show up in some scenarios.
    
    
    Thanks for the fix. 


> Validate That Planner Does Not Assume HashJoin Preserves Ordering for FS, MaprDB, or
Hive
> -----------------------------------------------------------------------------------------
>
>                 Key: DRILL-6089
>                 URL: https://issues.apache.org/jira/browse/DRILL-6089
>             Project: Apache Drill
>          Issue Type: Improvement
>    Affects Versions: 1.13.0
>            Reporter: Timothy Farkas
>            Assignee: Timothy Farkas
>            Priority: Major
>             Fix For: 1.13.0
>
>
> Explanation provided by Boaz:
> (As explained in the design document) The new "automatic spill" feature of the Hash-Join
operator may cause (if spilling occurs) the rows from the left/probe side to be returned in
a different order than their incoming order (due to splitting the rows into partitions).
> Currently the Drill planner assumes that left-order is preserved by the Hash-Join operator;
therefore if not changes, a query relying on that order may return wrong results (when the
Hash-Join spills).
> A fix is needed. Here are few options (ordered from the simpler down to the most complex):
>  # Change the order rule in the planner. Thus whenever an order is needed above (downstream)
the Hash-Join, the planner would add a sort operator. That would be a big execution time waste.
>  # When the planner needs the left-order above the Hash-Join, it may assess the size
of the right/build side (need statistics). If the right side is small enough, the planner
would set an option for the runtime to avoid spilling, hence preserving the left-side order.
In case spilling becomes necessary, the code would return an error (possibly with a message
suggesting setting some special option and retrying; the special option would add a sort operator
and allow the hash-join to spill).
>  # When generating the code for the fragment above the Hash-Join (where left-order should
be maintained) - at code-gen time check if the hash-join below spilled, and if so, add a sort
operator. (Nothing like that exists in Drill now, so it may be complicated).



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