Currently db.odbc.select and db.odbc.discovery have two mandatory parameters. One of them is data source name (DSN) and another one is just ignored when performing check:

The unique description will serve to identify the item in triggers etc.

It makes sense to store DSN in a separate dedicated field along with username and password. If db.odbc.discovery is obsoleted by ZBXNEXT-4086 we will only have one ODBC key possible and we can allow users to have meaningful keys without db.odbc.select prefix.