Extra Queries vs Added Controls

There is a difference between inserting additional fields via extra queries and simply adding controls (e.g. dropdown lists) yourself. In the former, a SQL statement is used to retrieve data from the database and/or to compute a value and is performed for each record. In the latter, the SQL statement (Value) only specifies which values to display (purely external), and is performed only once: when the window/control is created.

If you are using extra queries for computational purposes, you are advised to define them "conceptually" as virtual columns on logical views. The advantage is that they will automatically cascade onto the external schema, and that they can easily be accessed via ODBC.