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https://issues.apache.org/jira/browse/CALCITE-7463?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18071564#comment-18071564
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Zhen Chen commented on CALCITE-7463:
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Yes, relying on the physical ordering of data doesn't seem particularly
convincing, as the scanning process itself could be parallelized. The scenario
I was likely trying to describe is the following—though I'm not sure if this
specific issue actually arises in practice: a single `SORT` operation can be
followed by multiple `PROJECT` operations, and each `PROJECT` can, in turn, be
followed by multiple `SORT` operations. During the rule rewriting phase, the
system might not recurse deeply enough to fully inspect and account for this
specific structure. The plan might look something like this.
{code:java}
LogicalUnion(all=[false])
LogicalSort(fetch=[2])
LogicalProject(MGR=[$3], COMM=[$6])
LogicalSort(sort0=[$3], dir0=[ASC])
LogicalTableScan(table=[[CATALOG, SALES, EMP]])
LogicalSort(fetch=[2])
LogicalProject(MGR=[$3], COMM=[$6])
LogicalSort(sort0=[$3], dir0=[ASC])
LogicalTableScan(table=[[CATALOG, SALES, EMP]]) {code}
> 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
> 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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