join-performance problem

Hi,

I have problem using joins: there are indexes which could be used, but
sequential-scan takes place;
see:

>explain SELECT b.Kdnr, date_part('year', b.Datum), d.Name, e.Name, a.Menge
,
>     a.Rabproz, a.Netto - (a.netto * a.rabproz / 100),
>     a.Netto * b.kurs - ((a.netto * b.kurs) * a.rabproz / 100), 'RG'
>FROM Faktzeilen a, Fakt b, Artikel c, Kollektion d, ArGruppen e
>where a.Fanr = b.nr
>     and c.nr = a.Arnr
>     and c.Kollektion = d.Nr
>     and (c.Gruppe = e.nr or c.gruppe = 0)
>     and b.kdnr = 49736;

>NOTICE:  QUERY PLAN:996" 9L, 376C written
>
>Nested Loop  (cost=17.08..265.50 rows=30 width=152)
>  ->  Nested Loop  (cost=17.08..90.50 rows=5 width=115)
>        ->  Nested Loop  (cost=17.08..66.32 rows=5 width=78)
>              ->  Hash Join  (cost=17.08..42.14 rows=5 width=51)
>                    ->  Seq Scan on faktzeilen a  (cost=0.00..20.00
rows=1000 width=35)
>                    ->  Hash  (cost=17.07..17.07 rows=5 width=16)
>                          ->  Index Scan using fakt_tbkundentbfakt_key on
fakt b  (cost=0.00..17.07 rows=5 width=16)
>              ->  Index Scan using artikel_pkey on artikel c
(cost=0.00..4.82 rows=1 width=27)
>        ->  Index Scan using kollektion_pkey on kollektion d
(cost=0.00..4.82 rows=1 width=37)
>  ->  Seq Scan on argruppen e  (cost=0.00..20.00 rows=1000 width=37)
>
>EXPLAIN

my problem is the sequential scan for tabel faktzeilen (which has 250000
rows);
the following indexes exist:

primary index: fanr, znr
index2: fanr
index3: fanr, arnr, arname, pknr

can one tell me why index2 is not used for the join?

Thanks in advance

Wolfgang