László,

> I know, IB can't create index for a computed by column.
> I see, but WHY CAN'T?

because a computed column isn't stored and thus it cannot be indexed.
It seems to me that you are proposing persistent computed fields, which
would be a shortcut over what we have today (that is a real field with a
couple of triggers to keep it in sync), just less flexible (now you can
decide exactly under what conditions to recalculate a value in the
update trigger, whereas a persistent computed field would be
recalculated every time a record is written - unless the engine gets
very very smart).
Is there some other point I am missing?
Ciao
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