full outer join performance

Are full outer joins expected to perform much worse than inner joins?
I'm seeing 2 orders of magnitude difference for an almost identical
query. (Well, as "identical" as you can get, comparing a query with an
outer join to one without.) This is on 8.0.3, recently analyzed. Here
are the explain plans:

music=# explain select
music-#  extractbasenamefrompath(files.path),trms.trm
music-# from files,nodes
music-# full outer join trms on (trms.id = nodes.trm)
music-# where
music-#  nodes.fileid = files.id and
music-#  extractbasenamefrompath(files.path) = 'Joy Division/Substance/';
                                         QUERY PLAN
---------------------------------------------------------------------------------------------
 Hash Join  (cost=3932.55..11891.05 rows=18 width=117)
   Hash Cond: ("outer".fileid = "inner".id)
   ->  Hash Left Join  (cost=3867.51..11391.65 rows=86827 width=44)
         Hash Cond: ("outer".trm = "inner".id)
         ->  Seq Scan on nodes  (cost=0.00..1557.27 rows=86827 width=8)
         ->  Hash  (cost=2867.21..2867.21 rows=88521 width=44)
               ->  Seq Scan on trms  (cost=0.00..2867.21 rows=88521
width=44)
   ->  Hash  (cost=64.99..64.99 rows=18 width=81)
         ->  Index Scan using basename_idx on files  (cost=0.00..64.99
rows=18 width=81)
               Index Cond: (extractbasenamefrompath(path) = 'Joy
Division/Substance/'::text)
(10 rows)

music=# explain select
music-#  extractbasenamefrompath(files.path),trms.trm
music-# from
music-#  nodes,files,trms
music-# where
music-#  nodes.fileid = files.id and
music-#  nodes.trm = trms.id and
music-#  extractbasenamefrompath(files.path) = 'Joy Division/Substance/';
                                         QUERY PLAN
---------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.00..202.04 rows=18 width=117)
   ->  Nested Loop  (cost=0.00..119.46 rows=18 width=81)
         ->  Index Scan using basename_idx on files  (cost=0.00..64.99
rows=18 width=81)
               Index Cond: (extractbasenamefrompath(path) = 'Joy
Division/Substance/'::text)
         ->  Index Scan using nodes_fileid_idx on nodes
(cost=0.00..3.01 rows=1 width=8)
               Index Cond: (nodes.fileid = "outer".id)
   ->  Index Scan using trms_pkey on trms  (cost=0.00..4.57 rows=1 width=44)
         Index Cond: ("outer".trm = trms.id)
(8 rows)