Avoiding execution of some functions by query rewriting

Hello,

I would like to allow the execution of a function (my_function) only if its
argument (my_table.x) belongs to a predefined interval (e.g. [100,1000]).

Let's take the following query example :
(q) SELECT * FROM my_table WHERE my_function(mytable.x);

I would like this query automatically rewrites itself to check whether
"mytable.x" belong to the interval [100,1000] :
(q') SELECT * FROM my_table WHERE (my_table.x BETWEEN 100 AND 1000) AND
my_function(my_table.x);

The command EXPLAIN ANALYSE shows that the second query is really faster
than the first one.

How can I change the query execution plan in order to automate the process
of query rewriting (q into q') ?
Where can I store suitably the metadata about the interval [100,1000]
associated to my_function ?

Thanks by advance,

Thomas Girault