nil, Gnus will attempt to build old threads by fetching
more old headers—headers to articles marked as read. If you would
like to display as few summary lines as possible, but still connect as
many loose threads as possible, you should set this variable to
some or a number. If you set it to a number, no more than that
number of extra old headers will be fetched. In either case, fetching
old headers only works if the back end you are using carries overview
files—this would normally be
nnmaildir. Also remember that if the root of
the thread has been expired by the server, there’s not much Gnus can
do about that.
This variable can also be set to
invisible. This won’t have any
visible effects, but is useful if you use the A T command a lot
(see Finding the Parent).
The server has to support NOV for any of this to work.
This feature can seriously impact performance it ignores all locally
cached header entries. Setting it to
t for groups for a server
that doesn’t expire articles (such as news.gmane.org), leads to very
slow summary generation.
gnus-fetch-old-headers, but only used for ephemeral
Fetching old headers can be slow. A low-rent similar effect can be
gotten by setting this variable to
some. Gnus will then look at
References headers of all articles and try to string
together articles that belong in the same thread. This will leave
gaps in the threading display where Gnus guesses that an article
is missing from the thread. (These gaps appear like normal summary
lines. If you select a gap, Gnus will try to fetch the article in
question.) If this variable is
t, Gnus will display all these
“gaps” without regard for whether they are useful for completing the
thread or not. Finally, if this variable is
more, Gnus won’t cut
off sparse leaf nodes that don’t lead anywhere. This variable is
nil by default.
This is a rather obscure variable that few will find useful. It’s intended for those non-news newsgroups where the back end has to fetch quite a lot to present the summary buffer, and where it’s impossible to go back to parents of articles. This is mostly the case in the web-based groups.
If you don’t use those, then it’s safe to leave this as the default
nil. If you want to use this variable, it should be a regexp
that matches the group name, or
t for all groups.