>I guess you're right but I still would like to see an example of such index expression from your application.
>

well, i try.

It considers an customer interface, that it allows to define search expressions.
After to have inserted and validated these expressions,
it comes created a normalize function that returns the value of an expression as a string value.
Now, you suppose that these expressions, within a specified parameters set,
can select one primary keys.
Now, in the application you want to find all the expressions that
for the customer parameters set return a specific primary key within the searched range.
With the index of expressions values,
this can to be do faster because you can seek the range of expression
that return the pk range.

The search don't invalidate the customers expressions, but the customer parameters !

In truth the thing is more complex why every expression returns a range of values,
but hopes to have given the idea to you.