Inexplicable slow MySQL join query - mysql

Question

I have a pretty simple query that gives me headaches because of a very long execution time which I cannot explain. The query:
explain select A.* from NAV_ADRESSEN A left outer join MITGL_KENNZEICHEN K on (K.MNR=A.MNR)
where ((A.MNR='19012546') or (IMPORTID='19012546') or (K.KENNZEICHEN='19012546')) and
(not UNGUELTIG) order by ZUNAME, VORNAME limit 0, 25;
The query (the real one, not the explain) takes about 17 seconds, whether or not any matches are found.
The explain result:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE A index MNR,IMPORTID ZUNAME 164 NULL 25 Using where
1 SIMPLE K ref MNR MNR 23 gsco-test.A.MNR 1 Using where
This looks pretty normal to me. All relevant columns have keys (A.MNR, K.MNR, A.IMPORTID, K.KENNZEICHEN); the tables contain ~600 000 rows (NAV_ADRESSEN) and 180 rows (MITGL_KENNZEICHEN).
What could be the problem?
Edited to add:
The explain looks slightly different when leaving out the limit clause (but the execution time doubles):
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE A ALL MNR,IMPORTID NULL NULL NULL 544587 Using where; Using filesort
1 SIMPLE K ref MNR MNR 23 gsco-test.A.MNR 1 Using where
The table definition:
CREATE TABLE `MITGL_KENNZEICHEN` (
`PK` int(11) unsigned NOT NULL AUTO_INCREMENT,
`MNR` varchar(20) DEFAULT NULL,
`KENNZEICHEN` varchar(80) DEFAULT NULL,
`DESCRIPTION` varchar(80) DEFAULT NULL,
PRIMARY KEY (`PK`),
KEY `MNR` (`MNR`),
KEY `KENNZEICHEN` (`KENNZEICHEN`)
) ENGINE=InnoDB AUTO_INCREMENT=247 DEFAULT CHARSET=latin1;
... and ...
CREATE TABLE `NAV_ADRESSEN` (
`PK` int(11) NOT NULL AUTO_INCREMENT,
`MNR` varchar(20) NOT NULL,
-- ... 50 fields omitted for brevity ...
`UNGUELTIG` tinyint(1) NOT NULL,
`IMPORTID` varchar(20) NOT NULL,
`MATCHCODE` varchar(255) DEFAULT NULL,
PRIMARY KEY (`PK`),
UNIQUE KEY `MNR` (`MNR`),
KEY `ZUNAME` (`ZUNAME`,`VORNAME`),
KEY `IMPORTID` (`IMPORTID`),
KEY `MATCHCODE` (`MATCHCODE`),
KEY `ANGELEGTDAT` (`ANGELEGTDAT`)
) ENGINE=InnoDB AUTO_INCREMENT=1076829 DEFAULT CHARSET=latin1;

Answer 1

Just FYI, that query can (presumably) be rewritten as follows:
SELECT a.*
FROM nav_adressen a
JOIN mitgl_kennzeichen k
ON k.mnr = a.mnr
AND 19012546 IN (a.mnr,importid,k.kennzeichen)
AND NOT ungueltid
ORDER
BY zuname
, vorname
LIMIT 0,25;