> Great. So you store a type id separately. But you make that typeid define how to safely read (maybe not interpret, but still read) the data without needing to cross-reference to anything else.

Yes, that's how it works in Postgres

Unless the OP is completely wrong, it’s not. The author was able to fairly straightforwardly prepare a statement (thus selecting a type id), execute another statement (changing the meaning of that type id), and then execute the prepared statement (uses the old type id with the new semantics, which had a different idea of whether a particular field was a pointer).