group by: properly?

Hello,

I must confess I have always been a bit baffled by the GROUP BY, 
therefore I would appreciate if you could tell me if there's  a better way:

I have the table "items":
id           | integer | not null default nextval('items_id_seq'::text)seller_id    | integer | not null
====material     | integer | not nullitem_kind    | integer |finishing    | integer |
====amount       | integer | not null

What I need is to SELECT the list of items that have the same values in 
fields in between the ==== marks. 
The material, finishing and item_kind are references to the appropriate id 
fields in tables material, finishing and item_kinds: 

materials:
id SERIAL,
name_en text

and I would like to get the name of the material rather then the id from the 
query.

So I wrote:

SELECT M.name_en AS material, P.name_en AS itemtype, F.name_en 
AS finishing, amount FROM items I, item_kinds P, materials M, finish F 
WHERE F.id=I.finishing AND M.id=I.material AND P.id=I.item_kind 

This gives me the list of items with the names rather than id's already 
prepared, the list could be something like:

material itemtype finishing amount
'birch' 'SCAFFOLD BOARDS' 'levelled' 5
'birch' 'SCAFFOLD BOARDS' 'levelled' 33
'birch' 'SCAFFOLD BOARDS' 'levelled' 4
'oak' 'SCAFFOLD BOARDS' '' 7
'oak' 'C.L.S.' '' 66


Now, I want this query to give me only the one value for the items that differ 
only with the amount. so that i have:
'birch' 'SCAFFOLD BOARDS' 'levelled' 42
'oak' 'SCAFFOLD BOARDS' '' 7
'oak' 'C.L.S.' '' 66

The following GROUP BY accomplishes it:

SELECT M.name_en AS material, P.name_en AS itemtype, F.name_en 
AS finishing, sum(amount) FROM items I, item_kinds P, materials M, finish 
F WHERE F.id=I.finishing AND M.id=I.material AND P.id=I.item_kind 
GROUP BY M.name_en,P.name_en,F.name_en;

The question is:
assuming I will have quite a lot of rows in the result, will the performance be 
OK to GROUP BY the resulting text fields rather then by I.item_kind, 
I.material indexed numeric fields?

Is it possible to rewrite the query so that GROUP BY (or any alternative 
construct) deals with the numeric fields and yet I can receive the textual 
output (that I need) via a single query?


Any comments appreciated,

Emils