Use Screen Values
Prevent re-querying in typical routines if you can re-use a screen value.
You can block large queries, avoid unnecessary queries, or narrow queries down to required size by manual programming. An example is the Add Person Booking button in the Reservations window of the Travel Agency application, where a screen value from the parent record (the person booking the Reservation) is read, and written into the child record (person taking part in the Reservation as a Participant). No query is sent to the server.
When to use
In particular, in typical routines supported by customized windows, you may be able to use screen values and avoid re-querying altogether.
For example, use:
SqlScript(select :price - :down_payment "to be paid")
SqlScript(select price-paid "to be paid"