asolimando commented on code in PR #3264:
URL: https://github.com/apache/calcite/pull/3264#discussion_r1229907949
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core/src/main/java/org/apache/calcite/rel/metadata/BuiltInMetadata.java:
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@@ -105,6 +105,44 @@ interface Handler extends MetadataHandler<UniqueKeys> {
}
}
+ /**
+ * Metadata about which columns have foreign keys.
+ */
+ public interface ForeignKeys extends Metadata {
+ MetadataDef<ForeignKeys> DEF =
+ MetadataDef.of(ForeignKeys.class, ForeignKeys.Handler.class,
+ BuiltInMethod.FOREIGN_KEYS.method);
+
+ /**
+ * Determines the list of foreign keys for this expression. Foreign keys
are
+ * represented as an {@link org.apache.calcite.util.ImmutableBitSet}, where
+ * each bit position represents the column ordinal is foreign key.
+ *
+ * @param ignoreNulls if true, ignore null values when determining
+ * whether the keys are foreign keys
+ *
+ * @return set of foreign keys, or empty set if this information
+ * cannot be determined (whereas null indicates definitely
Review Comment:
Can you sketch an example for the "empty vs null" case here?
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