Being a newbie here myself I can't actually answer that, though if there isn't a session available already I think you're right in that it would probably be too much to add that layer of complexity.
In either case, my suggestion would only have held the sql query not the returned results and so the database would have been requeried anyway. I had that in mind to ensure the correctness of the search list (as far as possible). Maybe the querystring option is useful afterall, although I do remember that some browsers have a limit on the querystring the can send (in terms of length) and so a particularly complex query might end up truncated. Maybe sending the sql back as a POST would solve that, but it all depends on information I don't have yet (being new here).
Maybe someone else might know?
:o)
Dave
----Original Message Follows----
Can the query results be retained according to session? Is Koha using any kind
of stored sessions right now in the OPAC? If not, I'd hate to add another
layer of complexity unless the overhead savings were significant (in terms of
requerying the database). But it's a little beyond my skills to assess.