( resource result, int field_number )
pg_field_type_oid() returns an integer containing the
OID of the base type of the given field_number in the
given PostgreSQL result resource.
You can get more information about the field type by querying PostgreSQL's
pg_type system table using the OID obtained with this function.
The PostgreSQL format_type() function will convert a
type OID into an SQL standard type name.
If the field uses a PostgreSQL domain (rather than a basic type), it is
the OID of the domain's underlying type that is returned, rather than the OID
of the domain itself.