Optimize querry sql

Hi,

I want to optimize my qerry sql (execution time : 2448 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;

dns=> \d stat_dns_domaine;
Table «public.stat»

idxdxreseller | integer | not null
idxdo | integer | not null
idxd | integer | not null
nbrq | integer | default 0
month | 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_idxrevendeur» btree (idxrevendeur)
«stat_dns_domaine_idx_mois_annee_idxrev» btree (mois, annee, idxrevendeur)

\d revendeur;
Table «public.reseller»

idxreseller | integer | not null default
nextval(('idxrevendeur_seq'::text)::regclass)
namereseller | text |
asp | integer |
Index :
«reseller_pkey» PRIMARY KEY, btree (idxreseller)

Thank you

Stan