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https://issues.apache.org/jira/browse/BEAM-7049?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16933047#comment-16933047
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Rui Wang commented on BEAM-7049:
--------------------------------

Hmm this JIRA becomes more complicated than I imagined. 

So to conclude:

UnionMergeRule should work for UNION ALL somehow, but UNION will not work.

So our original intention(one shuffle to process all inputs) should only work 
for UNION ALL but not UNION, right?

> Merge multiple input to one BeamUnionRel
> ----------------------------------------
>
>                 Key: BEAM-7049
>                 URL: https://issues.apache.org/jira/browse/BEAM-7049
>             Project: Beam
>          Issue Type: Improvement
>          Components: dsl-sql
>            Reporter: Rui Wang
>            Assignee: sridhar Reddy
>            Priority: Major
>          Time Spent: 1h
>  Remaining Estimate: 0h
>
> BeamUnionRel assumes inputs are two and rejects more. So `a UNION b UNION c` 
> will have to be created as UNION(a, UNION(b, c)) and have two shuffles. If 
> BeamUnionRel can handle multiple shuffles, we will have only one shuffle



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