Xapian index not being used for message search in Roundcube

[Frederik Himpe]

I have configured Cyrus 3.0.13 with the Xapian search engine and
enabled search_fuzzy_always. This appears to work fine when I search in
the message body using the Evolution mail client, as I get a response
quickly:

<1582453709<L03163 UID SEARCH BODY linux
>1582453713>* SEARCH 226927
226929 226964 226974 226999 227215 227238 [...]
L03163 OK Completed (643
msgs in 0.970 secs)

However when I search messages using the Roundcube webmail client,
Roundcube does not get a response in time and shows no results. An
strace of the imapd proceess indicates it is STATing, OPENing and
MMAPing all files in the mailbox.

This is the log:
<1582455581<A0004 UID THREAD REFS US-ASCII ALL UNSEEN TEXT Linux
>1582455723>* THREAD
(229566)(229570)(229574)(229599)(229618)(229639)[...]
A0004 OK Completed (157 msgs in 11.340 secs)

So it appears Roundcube is using a different command to search. Is it
expected that this command does not use the Xapian search engine? Is
there a way to make it use it?

Some relevant snippets from imapd.conf:
sync_log: on
sync_log_channels: squatter

conversations: 1
search_engine: xapian
search_index_headers: no
search_batchsize: 8192
search_fuzzy_always: 1
defaultsearchtier: temp
tempsearchpartition-default: /var/lib/cyrus/search.temp
datasearchpartition-default: /var/lib/cyrus/search.data

cyrus.conf:

EVENTS {
        squatter1       cmd="/usr/bin/nice -n 19 /usr/sbin/cyrus squatter -z data -t temp,data" at=0517

}
DAEMON {
  squatter cmd="squatter -R"
}


Regards,

-- 
Frederik Himpe <frederik at frehi.be>