Optimize querry sql

Hi,

I had errors in my last emails. sorry

I want to optimize my query sql (execution time : 2665 ms) :

SELECT b.idxreseller, sum(a.nbrq), b.namereseller from stat a
,reseller b where b.asp=6 and a.idxreseller=b.reseller and
a.month=date_part('month',now() - interval '1 month') and
a.year=date_part('year',now() - interval '1 month') GROUP BY
b.idxreseller,b.namereseller limit 15;


Explain analyse :

Limit  (cost=1057.15..1057.16 rows=1 width=27) (actual
time=2655.083..2655.176 rows=15 loops=1)  ->  HashAggregate  (cost=1057.15..1057.16 rows=1 width=27) (actual
time=2655.074..2655.132 rows=15 loops=1)        ->  Nested Loop  (cost=0.00..1057.14 rows=1 width=27) (actual
time=0.646..2464.563 rows=18543 loops=1)              ->  Seq Scan on stat a  (cost=0.00..1042.98 rows=1
width=8) (actual time=0.273..1239.510 rows=24881 loops=1)                    Filter: (((month)::double precision =
date_part('month'::text, (now() - '1 mon'::interval))) AND
((year)::double precision = date_part('year'::text, (now() - '1
mon'::interval))))              ->  Index Scan using reseller_pkey on reseller b
(cost=0.00..14.15 rows=1 width=23) (actual time=0.034..0.038 rows=1
loops=24881)                    Index Cond: ("outer".idxrreseller = b.idxreseller)                    Filter: (asp =
6)Totalruntime: 2655.713 ms 




dns=> \d stat;                          Table «public.stat»
idxreseller | integer | not nullidxdo   | integer | not nullidxd       | integer | not nullnbrq         | integer |
default0month         | integer | default date_part('month'::text, (now() - 
'1 mon'::interval))year        | integer | default date_part('year'::text, (now() - '1
mon'::interval))

Index :  «stat_dns_domaine_idx_idxr_idxreseller» btree (dxreseller)  «stat_dns_domaine_idx_month_year_idxres» btree
(month,year, idxreseller) 




\d reseller;                                      Table «public.reseller»
idxreseller    | integer | not null default
nextval(('idxrevendeur_seq'::text)::regclass)namereseller     | text    |asp              | integer |
Index :  «reseller_pkey» PRIMARY KEY, btree (idxreseller)


Have you advices ?

Thank you

Stan