atomic function

Hello!

What's wrong with this function?

public | common_adviewnum_increase | integer | bigint,
character varying | postgres | plpgsql |
declare
row record;
result int;
begin
select into row viewnum from common_adviewnum where adid = $1
and site = $2 and day = now()::date;
if found then
result = row.viewnum + 1;
update common_adviewnum set viewnum = result where adid
= $1 and site = $2 and day = now()::date;
else
result = 1;
insert into common_adviewnum (adid, site, day, viewnum)
values ($1, $2, now()::date, result);
end if;
return result;
end;

Every 2-3 day I get this in the server log:

2005-08-12 19:08:43: ERROR: duplicate key violates unique constraint
"common_adviewnum_adid_site_day_index"
CONTEXT: SQL statement "insert into common_adviewnum (adid, site, day,
viewnum) values ( $1 , $2 , now()::date, $3 )"
PL/pgSQL function "common_adviewnum_increase" line 11 at SQL statement
select common_adviewnum_increase(820434,'H');

Mage