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https://issues.apache.org/jira/browse/CALCITE-3399?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16952392#comment-16952392
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Julian Hyde commented on CALCITE-3399:
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The current title "RelFieldTrimmer trim fields for UNION, INTERSECT, INTERSECT
ALL, EXCEPT, EXCEPT ALL" doesn't say whether trimming fields is desired or
undesired behavior. How about "Field-pruning for set operators (except UNION
ALL) changes query semantics"?
> RelFieldTrimmer trim fields for UNION, INTERSECT, INTERSECT ALL, EXCEPT,
> EXCEPT ALL
> -----------------------------------------------------------------------------------
>
> Key: CALCITE-3399
> URL: https://issues.apache.org/jira/browse/CALCITE-3399
> Project: Calcite
> Issue Type: Bug
> Components: core
> Reporter: jin xing
> Assignee: jin xing
> Priority: Major
> Labels: pull-request-available
> Time Spent: 1h 20m
> Remaining Estimate: 0h
>
> *RelFieldTrimmer#trimFields* provides functionality to trim fields for
> *UNION, UNION ALL, INTERSECT, INTERSECT ALL, EXCEPT, EXCEPT ALL;*
> But *UNION ALL, INTERSECT, INTERSECT ALL, EXCEPT, EXCEPT ALL* works by
> checking duplication. Column pruning on inputs of *SetOp* might lead to
> different semantics.
>
> Take below example for illustration
> {code:java}
> emp0:
> name, deptno
> "A", 0
> "B", 1
> emp1:
> name, deptno
> "C", 0
> "D", 2
> select deptno from
> (select name, deptno from emp0
> intersect
> select name, deptno from emp1)
> {code}
> Run above Sql on emp0 and emp1, result is not empty if trim fields on inputs
> of INTERSECT, but result is empty if DO NOT trim fields on inputs of INTERSECT
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