Developers quite commonly use "select firstonly recid from myTable where field = x" only to check whether a record does exist. When the recid is not the primary key of this table, SQL will build a query plan including a key lookup.
Example: "Select firstonly recid from custtable where accountnum == 123" will fist seek the index on accountnum, then do a lookup in the clustered index, to retrieve the recid.
My proposed solution would be to introduce a keyword for 'PrimaryKey' which will be replaced by the primary key field before contacting SQL. Example "Select firstonly PrimaryKey from custtable where accountnum == 123". The query processor should replace the keyword by the primary key (AccountNum) before contacting SQL.