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https://issues.apache.org/jira/browse/DRILL-4476?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15195029#comment-15195029
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Khurram Faraaz commented on DRILL-4476:
---------------------------------------
Verified on MapR Drill 1.7.0 commit ID : 050ff967
Empty input on left side of Union All.
{noformat}
0: jdbc:drill:schema=dfs.tmp> select columns[0] from `empty.csv` union all
select columns[0] from `vchar_f.csv`;
+----------+
| EXPR$0 |
+----------+
| 12345 |
| 1 |
| 0 |
| -1 |
| 200000 |
| 1000000 |
| 65535 |
| 13 |
| 19 |
| 17 |
| 11111 |
| 10101 |
+----------+
12 rows selected (0.538 seconds)
{noformat}
Empty input on left side of Union All.
{noformat}
0: jdbc:drill:schema=dfs.tmp> select key from `empty.json` union all select key
from `fewKeys.json`;
+------+
| key |
+------+
| 1 |
| 3 |
| 2 |
| 5 |
| 6 |
| 4 |
| 7 |
| 9 |
| 8 |
| 11 |
| 10 |
+------+
11 rows selected (0.34 seconds)
{noformat}
Empty input on both sides on Union All.
{noformat}
0: jdbc:drill:schema=dfs.tmp> select key from `empty.json` union all select key
from `empty.json`;
+------+
| key |
+------+
+------+
No rows selected (0.201 seconds)
{noformat}
> 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
> Fix For: 1.7.0
>
>
> 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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