asolimando commented on code in PR #3264:
URL: https://github.com/apache/calcite/pull/3264#discussion_r1229906537
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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
Review Comment:
Can you explain why we need this?
Maybe I am bit rusty on FK relationships, but I guess that `unknownAsFalse`
semantics will apply (like for `WHERE` clauses), so there is no way a row can
"point" to a `NULL` value in the parent table.
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