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https://issues.apache.org/jira/browse/CALCITE-7431?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18063958#comment-18063958
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Zhen Chen commented on CALCITE-7431:
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[~xuyangzhong] I understand, but currently RelTraitSet is deeply embedded,
making it difficult for the current architecture to be compatible with both. If
you have any good ideas, please feel free to discuss and learn together.
> 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
> Labels: pull-request-available
>
> 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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