Query Q49 (degree of parallelism: 4)

select  channel, item, return_ratio, return_rank, currency_rank from
 (select
 'web' as channel
 ,web.item
 ,web.return_ratio
 ,web.return_rank
 ,web.currency_rank
 from (
 	select 
 	 item
 	,return_ratio
 	,currency_ratio
 	,rank() over (order by return_ratio) as return_rank
 	,rank() over (order by currency_ratio) as currency_rank
 	from
 	(	select ws.ws_item_sk as item
 		,(cast(sum(coalesce(wr.wr_return_quantity,0)) as decimal(15,4))/
 		cast(sum(coalesce(ws.ws_quantity,0)) as decimal(15,4) )) as return_ratio
 		,(cast(sum(coalesce(wr.wr_return_amt,0)) as decimal(15,4))/
 		cast(sum(coalesce(ws.ws_net_paid,0)) as decimal(15,4) )) as currency_ratio
 		from 
 		 web_sales ws left outer join web_returns wr 
 			on (ws.ws_order_number = wr.wr_order_number and 
 			ws.ws_item_sk = wr.wr_item_sk)
                 ,date_dim
 		where 
 			wr.wr_return_amt > 10000 
 			and ws.ws_net_profit > 1
                         and ws.ws_net_paid > 0
                         and ws.ws_quantity > 0
                         and ws_sold_date_sk = d_date_sk
                         and d_year = 2001
                         and d_moy = 12
 		group by ws.ws_item_sk
 	) in_web
 ) web
 where 
 (
 web.return_rank <= 10
 or
 web.currency_rank <= 10
 )
 union
 select 
 'catalog' as channel
 ,catalog.item
 ,catalog.return_ratio
 ,catalog.return_rank
 ,catalog.currency_rank
 from (
 	select 
 	 item
 	,return_ratio
 	,currency_ratio
 	,rank() over (order by return_ratio) as return_rank
 	,rank() over (order by currency_ratio) as currency_rank
 	from
 	(	select 
 		cs.cs_item_sk as item
 		,(cast(sum(coalesce(cr.cr_return_quantity,0)) as decimal(15,4))/
 		cast(sum(coalesce(cs.cs_quantity,0)) as decimal(15,4) )) as return_ratio
 		,(cast(sum(coalesce(cr.cr_return_amount,0)) as decimal(15,4))/
 		cast(sum(coalesce(cs.cs_net_paid,0)) as decimal(15,4) )) as currency_ratio
 		from 
 		catalog_sales cs left outer join catalog_returns cr
 			on (cs.cs_order_number = cr.cr_order_number and 
 			cs.cs_item_sk = cr.cr_item_sk)
                ,date_dim
 		where 
 			cr.cr_return_amount > 10000 
 			and cs.cs_net_profit > 1
                         and cs.cs_net_paid > 0
                         and cs.cs_quantity > 0
                         and cs_sold_date_sk = d_date_sk
                         and d_year = 2001
                         and d_moy = 12
                 group by cs.cs_item_sk
 	) in_cat
 ) catalog
 where 
 (
 catalog.return_rank <= 10
 or
 catalog.currency_rank <=10
 )
 union
 select 
 'store' as channel
 ,store.item
 ,store.return_ratio
 ,store.return_rank
 ,store.currency_rank
 from (
 	select 
 	 item
 	,return_ratio
 	,currency_ratio
 	,rank() over (order by return_ratio) as return_rank
 	,rank() over (order by currency_ratio) as currency_rank
 	from
 	(	select sts.ss_item_sk as item
 		,(cast(sum(coalesce(sr.sr_return_quantity,0)) as decimal(15,4))/cast(sum(coalesce(sts.ss_quantity,0)) as decimal(15,4) )) as return_ratio
 		,(cast(sum(coalesce(sr.sr_return_amt,0)) as decimal(15,4))/cast(sum(coalesce(sts.ss_net_paid,0)) as decimal(15,4) )) as currency_ratio
 		from 
 		store_sales sts left outer join store_returns sr
 			on (sts.ss_ticket_number = sr.sr_ticket_number and sts.ss_item_sk = sr.sr_item_sk)
                ,date_dim
 		where 
 			sr.sr_return_amt > 10000 
 			and sts.ss_net_profit > 1
                         and sts.ss_net_paid > 0 
                         and sts.ss_quantity > 0
                         and ss_sold_date_sk = d_date_sk
                         and d_year = 2001
                         and d_moy = 12
 		group by sts.ss_item_sk
 	) in_store
 ) store
 where  (
 store.return_rank <= 10
 or 
 store.currency_rank <= 10
 )
 ) t1
 order by 1,4,5,2
 limit 100;

Measured execution times:
4000MB: 1866
4000MB-sp(lt0.2/ts5/min10/max100): 1860
8000MB: 1879
8000MB-sp(lt0.2/ts5/min10/max100): 1857