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https://issues.apache.org/jira/browse/DRILL-3805?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14876616#comment-14876616
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Daniel Barclay (Drill) commented on DRILL-3805:
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Seems likely to be related to DRILL-2288 and DRILL-3659.
> Empty JSON on LHS UNION non empty JSON on RHS must return results
> -----------------------------------------------------------------
>
> Key: DRILL-3805
> URL: https://issues.apache.org/jira/browse/DRILL-3805
> Project: Apache Drill
> Issue Type: Improvement
> Components: Execution - Relational Operators
> Affects Versions: 1.2.0
> Reporter: Khurram Faraaz
> Assignee: Sean Hsuan-Yi Chu
> Priority: Minor
>
> When the input on LHS of UNION operator is empty and there is non empty input
> on RHS of Union, we need to return the data from the RHS. Currently we return
> SchemaChangeException.
> {code}
> 0: jdbc:drill:schema=dfs.tmp> select key1 from `empty.json` UNION select key1
> from `fewRows.json`;
> Error: SYSTEM ERROR: SchemaChangeException: The left input of Union-All
> should not come from an empty data source
> Fragment 0:0
> [Error Id: f0fcff87-f470-46a8-9733-316b7da1a87f on centos-02.qa.lab:31010]
> (state=,code=0)
> {code}
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