Gnus: caching message headers?

[Gregory Heytings]

Hi list,

I'm trying to configure Gnus to access my email through IMAP.

I'm too accustomed to traditional MUAs to switch to the "Gnus way" in one 
step, so I have:

(setq gnus-permanently-visible-groups ".*")
(setq gnus-parameters
   '((".*"
      (gnus-show-threads nil)
      (gnus-use-scoring nil)
      (gnus-article-sort-functions '(gnus-article-sort-by-number))
      (display . all))))

I would like to have, in the *Summary*, a complete list of the emails 
contained in a folder when I hit RET on its label.  Each time I do this 
however, Gnus asks me how many articles I want to retrieve, and issues a 
"UID FETCH 1:N (UID RFC822.SIZE BODYSTRUCTURE BODY.PEEK[HEADER.FIELDS 
(...)])" IMAP command, which can take quite some time to complete when N 
is large.

Is there a way to convince Gnus to cache the result of that command 
(without caching all emails), and to issue a command only for the new 
UIDs?  Caching the result of that command should not eat too much disk 
space.

I apologize in advance if that question is naïve, or if I missed some part 
of the manual.

Gregory