ASF GitHub Bot commented on DRILL-6089:

Github user amansinha100 commented on the issue:

    @ilooner thanks for making the changes.  I have a minor comment on the test 
    Otherwise LGTM.  +1.  

> 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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