How can you have an Exclusive Lock on nothing? please help me understand this lock monitoring query output

Hi.  I use the following query (from
http://wiki.postgresql.org/wiki/Lock_Monitoring)
to monitor locks; and I've got an ExlusiveLock that does not have a relation
name associated with it.  What is locked with the Exclusive Lock in this case,
please?  (it's between "d" and "e" tables below)


psql -U postgres -d ddcKeyGen -c 'select
     pg_class.relname,pg_locks.transactionid, pg_locks.mode, pg_locks.granted,
     pg_stat_activity.usename,substr(pg_stat_activity.current_query,1,50),
pg_stat_activity.query_start,
     age(now(),pg_stat_activity.query_start) as "age", pg_stat_activity.procpid
   from pg_stat_activity,pg_locks left
     outer join pg_class on (pg_locks.relation = pg_class.oid)
   where pg_locks.pid=pg_stat_activity.procpid order by query_start;'



Output:

 a_index                       |               | AccessShareLock
   | t       | user |
  | 2011-04-14 17:36:01.257669-07 | 00:21:28.847825 |   16798
 b_index                     |               | AccessShareLock
 | t       | user |
| 2011-04-14 17:36:01.257669-07 | 00:21:28.847825 |   16798
 c_index                     |               | AccessShareLock
 | t       | user |
| 2011-04-14 17:36:01.257669-07 | 00:21:28.847825 |   16798
 d                                        |               |
AccessShareLock          | t       | user |
                        | 2011-04-14 17:36:01.257669-07 |
00:21:28.847825 |   16798
                                                       |
| ExclusiveLock            | t       | user |
                          | 2011-04-14 17:36:01.257669-07 |
00:21:28.847825 |   16798
 e                                               |               |
AccessShareLock          | t       | user |
                        | 2011-04-14 17:36:01.257669-07 |
00:21:28.847825 |   16798
 f_index                     |               | ShareLock
 | t       | user |
| 2011-04-14 17:36:01.257669-07 | 00:21:28.847825 |   16798
 g_index             |               | AccessShareLock          | t
   | user |                                                    |
2011-04-14 17:36:01.257669-07 | 00:21:28.847825 |   16798


Thanks,
Aleksey