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https://issues.apache.org/jira/browse/CALCITE-7463?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18073399#comment-18073399
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Alessandro Solimando commented on CALCITE-7463:
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[~sphillips], another example involving an ORDER BY was proposed here, but we
it's still not clear to me if this one is problematic either. I agree with your
comment, though, as long as we pick one legal alternative and we reduce cost,
the planner it's doing its job.
Thanks [~zabetak] for pointing to CALCITE-4160, which has a lot of interesting
details.
I re-took a look at the rule details, and I am pretty convinced that there is
no issue with this rule, as per the SQL standard, unless we can find a concrete
counter-example, I think we can close with "Not a bug" for now and revisit if
any problem surfaces later, wdyt [~jensen]?
> UnionToFilterRule incorrectly rewrites UNION with LIMIT
> -------------------------------------------------------
>
> Key: CALCITE-7463
> URL: https://issues.apache.org/jira/browse/CALCITE-7463
> Project: Calcite
> Issue Type: Bug
> Components: core
> Affects Versions: 1.41.0
> Reporter: Zhen Chen
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.42.0
>
>
> The {{UnionToFilterRule}} produces incorrect results when applied to inputs
> that contain {{{}LIMIT{}}}.
> Specifically, the rule incorrectly collapses:
> {code:java}
> (SELECT mgr, comm FROM emp LIMIT 2)
> UNION
> (SELECT mgr, comm FROM emp LIMIT 2) {code}
> into:
> {code:java}
> SELECT DISTINCT mgr, comm FROM emp LIMIT 2 {code}
> This transformation is {*}not semantically equivalent{*}.
> *Reproduction*
> SQL
> {code:java}
> (SELECT mgr, comm FROM emp LIMIT 2)
> UNION
> (SELECT mgr, comm FROM emp LIMIT 2) {code}
> h4. Plan Before
> {code:java}
> LogicalUnion(all=[false])
> LogicalSort(fetch=[2])
> LogicalProject(MGR=[$3], COMM=[$6])
> LogicalTableScan(table=[[CATALOG, SALES, EMP]])
> LogicalSort(fetch=[2])
> LogicalProject(MGR=[$3], COMM=[$6])
> LogicalTableScan(table=[[CATALOG, SALES, EMP]]) {code}
> *Plan After (Incorrect)*
> {code:java}
> LogicalAggregate(group=[{0, 1}])
> LogicalSort(fetch=[2])
> LogicalProject(MGR=[$3], COMM=[$6])
> LogicalTableScan(table=[[CATALOG, SALES, EMP]]) {code}
> *Expected Behavior*
> The transformation should NOT be applied when any input of UNION contains
> LogicalSort(That contains ORDER BY, LIMIT, OFFSET).
>
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