xiedeyantu commented on code in PR #4745:
URL: https://github.com/apache/calcite/pull/4745#discussion_r2692365981
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core/src/main/java/org/apache/calcite/plan/RelTraitSet.java:
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@@ -389,7 +389,14 @@ public RelTraitSet getDefaultSansConvention() {
*/
@SuppressWarnings("unchecked")
public <T extends RelCollation> @Nullable T getCollation() {
- return (@Nullable T) getTrait(RelCollationTraitDef.INSTANCE);
+ RelTrait trait = getTrait(RelCollationTraitDef.INSTANCE);
+ if (trait instanceof RelCompositeTrait) {
+ // If the trait is a composite trait, we return the first one as a
+ // representative trait. If RelCompositeTrait contain [[a, b], [a]].
+ // Selects [a,b] as it implies [a]; preserves the strongest ordering.
+ return (T) ((RelCompositeTrait) trait).trait(0);
Review Comment:
You're right, that's not rigorous, but returning only one collation here is
indeed a bit radical. I think the design of RelCompositeTrait hasn't been used
very well. I couldn't think of a better way to change it.
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