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https://issues.apache.org/jira/browse/CALCITE-7463?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18073355#comment-18073355
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Steven Phillips commented on CALCITE-7463:
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I'm still of the opinion that this rewrite is safe, and I haven't seen any
examples where this rewrite gives an incorrect result. The fact that the
rewrite narrows the space of possible correct results to a smaller space of
correct results does not make the results incorrect. Part of my reasoning is
based on the fact that, given a LIMIT without an ORDER BY, there are no
expectations about which rows get returned, so the optimizer making a choice to
return rows in a way that satisfy the limit but also reduce the cost of the
query is in fact a good thing.
I want to know what the real problem we're trying to solve is. Just one example
where this rule results in an incorrect result, or even undesirable plan.
> 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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