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Date: Wed, 9 Mar 2016 22:54:40 +0000 (UTC)
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Subject: [jira] [Commented] (DRILL-4476) Enhance Union-All operator for
 dealing with empty left input or empty both inputs
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    [ https://issues.apache.org/jira/browse/DRILL-4476?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15188252#comment-15188252 ] 

ASF GitHub Bot commented on DRILL-4476:
---------------------------------------

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

    https://github.com/apache/drill/pull/407#discussion_r55605493
  
    --- Diff: exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/union/UnionAllRecordBatch.java ---
    @@ -491,6 +556,25 @@ private void inferOutputFieldsFromLeftSide() {
           }
         }
     
    +    private void inferOutputFieldsFromRightSide() {
    --- End diff --
    
    Inferencing happens only once when Drill receives the very first batches from left and right. (Schema-change is not yet supported for Union-All).
    
    Let me summarize the inference in four different situations:
    First of all, the field names are always determined by the left side (even when the left side is from empty file, we have the column names. Please see the comment above.)
    
    1. Left: non-empty; Right: non-empty=> types determined by both sides with implicit casting involved
    2. Left: empty; Right: non-empty=> type from the right
    3. Left: non-empty; Right: empty=> types from the left
    4. Left: empty; Right: empty=> types are nullable integer


> Enhance Union-All operator for dealing with empty left input or empty both inputs
> ---------------------------------------------------------------------------------
>
>                 Key: DRILL-4476
>                 URL: https://issues.apache.org/jira/browse/DRILL-4476
>             Project: Apache Drill
>          Issue Type: Bug
>          Components: Execution - Relational Operators
>            Reporter: Sean Hsuan-Yi Chu
>            Assignee: Sean Hsuan-Yi Chu
>
> Union-All operator does not deal with the situation where left side comes from empty source.
> Due to DRILL-2288's enhancement for empty sources, Union-All operator now can be allowed to support this scenario.


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