Full text search indexes not used for header/body OR queries?

[Robert L Mathews]

I've noticed that when using Lucene full text search, most queries use
the indexes and/or header cache and are fast:

. SEARCH BODY test
. OK Search completed (0.001 secs).

. SEARCH SUBJECT test
. OK Search completed (0.053 secs).

. SEARCH BODY test SUBJECT test
. OK Search completed (0.002 secs).

. SEARCH OR SUBJECT test FROM test
. OK Search completed (0.093 secs).

But an OR query that mixes headers and body does not use the available
FTS indexes for the BODY part and is slow:

. SEARCH OR BODY test SUBJECT test
* OK Searched 62% of the mailbox, ETA 0:05
* OK Searched 70% of the mailbox, ETA 0:04
. OK Search completed (15.147 secs).

Is this the expected behavior? Since the FTS code can handle an AND of
header and body searches, I'm surprised it doesn't do the same for an OR.

I noticed this while tracking down poor performance in Thunderbird,
which issues searches like this:

UID SEARCH RETURN (ALL) (OR FROM "Evelyn" (OR SUBJECT "Evelyn" (OR TO
"Evelyn" (OR CC "Evelyn" BODY "Evelyn")))) NOT DELETED

These are slow even with FTS enabled because of this behavior.

I'm using Dovecot 2.1.7 from Debian wheezy. (I know this is outdated;
however, I've examined the 2.1.x and 2.2.x changelogs and found no
mention of it.)

-- 
Robert L Mathews, Tiger Technologies, http://www.tigertech.net/