UNRESOLVED_OID

The OID standing for "no type resolved".

PostgreSQL never assigns 0 to a real type, so it is safe to use as the absence marker. It is what a converter sees as its expectedOid for a top-level parameter, since the driver does not ask the server what it wants: a type is known up front only where the value carries one - wrapped in PgTyped, or a PgContainer holding its own OID - or where the value is nested inside a composite or array whose attribute and element types the catalog already knows.

Unresolved is not the same as undeclared. A value that reaches the wire without a type resolved is declared from the codec registered for its Kotlin class, so a String goes out as text; 0 itself is only ever sent as the parameter type for a bare null, which is what makes an untyped null ambiguous to the server.