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https://issues.apache.org/jira/browse/DRILL-6089?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16361485#comment-16361485
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ASF GitHub Bot commented on DRILL-6089:
---------------------------------------

Github user ilooner commented on a diff in the pull request:

    https://github.com/apache/drill/pull/1117#discussion_r167694158
  
    --- Diff: 
exec/java-exec/src/test/java/org/apache/drill/exec/physical/impl/join/TestHashJoinAdvanced.java
 ---
    @@ -197,4 +198,24 @@ public void emptyPartTest() throws Exception {
           BaseTestQuery.resetSessionOption(ExecConstants.SLICE_TARGET);
         }
       }
    +
    +  @Test // DRILL-6089
    +  public void testJoinOrdering() throws Exception {
    +    final String query = "select * from dfs.`sample-data/nation.parquet` 
nation left outer join " +
    +      "(select * from dfs.`sample-data/region.parquet`) " +
    +      "as region on region.r_regionkey = nation.n_nationkey order by 
region.r_name desc";
    +    final String plan = getPlanInString("EXPLAIN PLAN for " + 
QueryTestUtil.normalizeQuery(query), OPTIQ_FORMAT);
    +    lastSortAfterJoin(plan);
    --- End diff --
    
    The method `testPlanMatchingPatterns(String query, String[] 
expectedPatterns, String[] excludedPatterns)` is not sufficient out of the box 
since I have to add the `Pattern.DOTALL` flag when compiling the pattern in 
order to have the regex match across new lines. So I've added 
`testPlanMatchingPatterns(String query, Pattern[] expectedPatterns, Pattern[] 
excludedPatterns)`


> 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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