Indexes won't work if using DISTINCT

[Andrey M]

I have two tables:

1.
product_id int(11)
product_name varchar(64)
product_update_date int(11)
product_parameters_md5 varchar(32)
part_id varchar(255)

Indexes:

product_parameters_md5 UNIQUE product_parameters_md5
index_product_id INDEX product_id
index_product_name INDEX product_name
index_product_update_date INDEX product_update_date
index_part_id INDEX part_id

2. id int(11) auto_increment
iproduct_id int(11) icategory_id int(11) imanufacturer_id int(11) idistributor_id int(11) product_name varchar(64)
quantity int(11) packaging varchar(255)
wholesale_price decimal(12,2) s_details varchar(255)
details text
sell_in_online_store int(1)
special_offer int(1) special_offer_details text
posted datetime
active int(1) premier int(11) part_alert int(1) cache_status int(1) product_parameters_md5 varchar(32)

Indexes:

PRIMARY PRIMARY id
idx_c INDEX icategory_id
idx_m INDEX imanufacturer_id
idx_date INDEX posted
idx_d INDEX idistributor_id
idx_md5 INDEX product_parameters_md5

I have a query joining these tables:

SELECT DISTINCT cdn.product_id, cdn.product_parameters_md5, cdn.product_name, cdn.part_id, cdn.product_update_date
FROM jos_dn_cache_inventory_partid_asc cdn, jos_dn_inventory dn
WHERE cdn.product_parameters_md5 = dn.product_parameters_md5
AND dn.quantity >0
AND dn.icategory_id =1
ORDER BY cdn.part_id
LIMIT 0 , 30

The problem here is the ORDER BY cdn.part_id without sorting or without Distinct the query works thousand times faster but I need to have both.

id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra
1|SIMPLE|cdn|ALL|product_parameters_md5|NULL|NULL|NULL|35018|Using temporary; Using filesort
1|SIMPLE|dn|ref|idx_c,idx_md5|idx_md5|34|cdn.product_parameters_md5| 1 |Using where; Distinct

If you look closer the first table has primary index product_parameters_md5 and it would usually make sense to add the part_id field to that index but the primary index cannot consist of more than one field.

Please advise how to solve this issue. Thanks!