FAQs

A virtual table is a table who's rows are rows from other parts of the database.  Which rows are included in the table is controlled by a calculation which produces a list of rows as an output instead of a number or a more conventional result.  A simple virtual table calculation could be Filter ( Houses , Price>400000 )
this would return a list of all the houses in the Houses list who's Price was greater than 400,000

When you enter such a calculation the interface gives you the option of displaying the result as a virtual table. The individual cells of a virtual table can be edited and rows can be deleted but new rows cannot be added to a virtual table.

Every time you make a change to the database every visible calculated value is recalculated.

The display will slow down if you have calculations which go through a lot of data. 

There are two things you can do about this.

Firstly, use intermediate values.  If a column has a calculation which contains a summary of another column (e.g. a total) then that summary will have to be carried out for every row. You could be calculating the same summary value over and over again. If the column is 10s of thousands of rows long this could be very wasteful. To get round this create an intermediate calculation on the page which summarizes the column.  Then refer to this value in the column expression.  The intermediate value will only be calculated once and then referred to over and over again much more quickly.

Secondly, disable auto recalculation. using the 'View-View-Auto recalculation mode' menu option.  Whenever a recalculation is missed the program will display a button in the top right hand corner which can be used to bring the display up to date. 
Auto recalculation is controlled per view so you can leave it permanently disabled for
a particularly slow view.