xiedeyantu commented on code in PR #4745:
URL: https://github.com/apache/calcite/pull/4745#discussion_r2692326817
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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:
I think so; its constructor has a special sorting logic.
```
private RelCompositeTrait(RelTraitDef traitDef, T[] traits) {
this.traitDef = traitDef;
this.traits = requireNonNull(traits, "traits");
//noinspection unchecked
assert Ordering.natural()
.isStrictlyOrdered(Arrays.asList((Comparable[]) traits))
: Arrays.toString(traits);
for (T trait : traits) {
assert trait.getTraitDef() == this.traitDef;
}
}
```
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