Update with Subquery Performance

We have a large datawarehouse stored in postgres and temp tables are created
based on user query. The process of temp table creation involves selecting
data from main fact table, this includes several select and update
statements and one of the following update statement is having performance
issues.

The newly temp table created for this scenario contains 22712 rows. Here is
the query

alter table dummy add column gp numeric(40,15);
update dummy set gp=(select (
case when sum(temp.pd) <> 0 then sum(temp.gd)/sum(temp.pd)*100 else 0 end
) from dummy as temp
where temp.product=dummy.product)

Now this query basically updates a table using values within itself in the
subquery but it takes tooooo much time i.e. approx 5 mins. The whole temp
table creation process is stucked in this query (there are 4 additional such
updates with same problem). Index creation is useless here since its only a
one time process.

Here is the strip down version (the part making performance issue) of above
query i.e. only select statement
-------------------------------
select (case when sum(temp.pd) <> 0 then sum(temp.gd)/sum(temp.pd)*100 else
0 end ) from dummy as temp, dummy as temp2
where temp.product=temp2.product group by temp.product

"HashAggregate (cost=1652480.98..1652481.96 rows=39 width=39)"
" -> Hash Join (cost=1636.07..939023.13 rows=71345785 width=39)"
" Hash Cond: (("temp".product)::text = (temp2.product)::text)"
" -> Seq Scan on dummy "temp" (cost=0.00..1311.03 rows=26003
width=39)"
" -> Hash (cost=1311.03..1311.03 rows=26003 width=21)"
" -> Seq Scan on dummy temp2 (cost=0.00..1311.03 rows=26003
width=21)"
-------------------------------

Whats the solution of this problem, or any alternate way to write this
query?