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https://issues.apache.org/jira/browse/CALCITE-7431?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18063146#comment-18063146
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Zhen Chen commented on CALCITE-7431:
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To be honest, I'm not entirely clear on the logic here either, but I do think
there's a lack of compatibility. I originally intended to use RelCompositeTrait
to support carrying multiple sorting attributes and then eliminate redundant
sorting attributes at the planner level using functional dependencies (see
CALCITE-7234 for details), but I haven't found a good implementation. Perhaps
the problem described in Jira should be clarified.
> RelTraitSet#getTrait seems to mishandle RelCompositeTrait
> ---------------------------------------------------------
>
> Key: CALCITE-7431
> URL: https://issues.apache.org/jira/browse/CALCITE-7431
> Project: Calcite
> Issue Type: Bug
> Components: core
> Affects Versions: 1.41.0
> Reporter: Xuyang Zhong
> Priority: Major
>
> Below is my test and an exception will be thrown:
>
> {code:java}
> RelBuilder b = RelBuilder.create(RelBuilderTest.config().build());
> RelNode in = b
> .scan("EMP")
> .sort(3) // MGR asc
> .project(b.field(3), b.alias(b.field(3), "MGR2")) // MGR, MGR as MGR2
> .build();
> // the following 3 lines will all fail
> System.err.println(in.getTraitSet().getCollation());
> System.err.println(in.getTraitSet().getCollations());
> System.err.println((RelCollation)
> in.getTraitSet().getTrait(RelCollationTraitDef.INSTANCE));
> // the exception:
> java.lang.ClassCastException: org.apache.calcite.plan.RelCompositeTrait
> cannot be cast to org.apache.calcite.rel.RelCollation{code}
> The reason is that at `{{{}return (T) getTrait(index);{}}}` in
> `RelTraitSet#getTrait`, the trait we actually get is a
> `{{{}RelCompositeTrait`{}}}, but it is cast to `{{{}RelCollation`{}}}, which
> leads to a Java cast error.
> I’m not sure whether this is by design: Callers of
> `{{{}RelTraitSet#getTrait`{}}} have to be aware that the input trait may have
> multiple values (`{{{}RelMultipleTrait`{}}}), and therefore should use
> `{{{}#getTraits`{}}} instead. From my perspective, relying on callers to be
> aware of this is not ideal. At the very least, `{{{}#getTrait{}}}` should
> validate whether the trait is a `{{{}RelMultipleTrait{}}}` and explicitly
> indicate that `{{{}#getTraits`{}}} should be used instead.
> I encountered this issue because for the SQL below, in older versions,
> `{{{}FilterProjectTransposeRule#Line187`{}}} would throw this error. Although
> it was fixed in
> [https://github.com/apache/calcite/pull/4704/changes#r2644415395], I found
> that there still seem to be many places that call `{{{}#getTrait`{}}}
> incorrectly—for example, `{{{}#getCollation()`{}}},
> `{{{}#getCollations()`{}}}, and the handling of `{{{}RelDistribution`{}}}.
> {code:java}
> SELECT
> *
> FROM (
> SELECT
> task_status, task_status + 1
> FROM src
> WHERE
> task_date >= '2026-02-01' AND task_date < '2026-02-01'
> -- this will convert the source to empty value source,
> -- and empty value source will output multi collations
> )
> WHERE
> task_status > 10 {code}
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