A foreign key constraint in postgres requires that the entire column value in the foreign key match a corresponding column value of a key in the referenced doma

A foreign key constraint in postgres requires that the entire column value in the foreign key match a corresponding column value of a key in the referenced domain table. Unless there have been quite recent extensions to the DDL in postgres, there is no foreign key constraint syntax to express a REFERENCES constraint over parts of a structured column. This is not supported for even the simple case of constraining one field in a UDT/composite type, much less the harder problem of constraining a variable cardinality set of elements in an array, or keys/values within a json/jsonb document. You can try to emulate it with triggers, and start to appreciate the subtle complexities of the problem. For example, would we want a new class of element-wise actions to act analogous to ON DELETE/UPDATE CASCADE/SET NULL? Rather than pruning referenced row or referencing column value, you'd want to mutate just an element within the structured type, right? How would you expose these many choices if trying to design a new constraint syntax with reusable machinery?