indexes on functions, then modifying functions

I have a plpgsql function that I am generating from time to time, like
once a week or so. What it does is it compares some phone numbers for
certain prefixes and then outputs the phone numbers without the
prefixes.

First I tried to make the function read from the database the prefixes,
but when I tried indexing it the whole database crashed (actually it
didn't crash, it just ate all resources on the computer and thus
crippled it).

So the solution I opted for is to recreate the function from time to
time, considering that there is a small number of such prefixes to be
removed.

My question is: what happends if I index this function, then I change
it? Does the index automatically recomputes itself or do I have to
reindex manually?

Thank you!

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