mihaibudiu commented on code in PR #4745:
URL: https://github.com/apache/calcite/pull/4745#discussion_r2692376332
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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:
But you have to implement "satisfies".
I wonder whether checking all traits in a set would work - if any of them
satisfies the requested collation.
Then you don't need to worry about which one is "the most general"
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