Query Q5 (degree of parallelism: 4)

with ssr as
 (select s_store_id,
        sum(sales_price) as sales,
        sum(profit) as profit,
        sum(return_amt) as returns,
        sum(net_loss) as profit_loss
 from
  ( select  ss_store_sk as store_sk,
            ss_sold_date_sk  as date_sk,
            ss_ext_sales_price as sales_price,
            ss_net_profit as profit,
            cast(0 as decimal(7,2)) as return_amt,
            cast(0 as decimal(7,2)) as net_loss
    from store_sales
    union all
    select sr_store_sk as store_sk,
           sr_returned_date_sk as date_sk,
           cast(0 as decimal(7,2)) as sales_price,
           cast(0 as decimal(7,2)) as profit,
           sr_return_amt as return_amt,
           sr_net_loss as net_loss
    from store_returns
   ) salesreturns,
     date_dim,
     store
 where date_sk = d_date_sk
       and d_date between cast('2000-08-23' as date) 
                  and date_add(cast('2000-08-23' as date), 14)
       and store_sk = s_store_sk
 group by s_store_id)
 ,
 csr as
 (select cp_catalog_page_id,
        sum(sales_price) as sales,
        sum(profit) as profit,
        sum(return_amt) as returns,
        sum(net_loss) as profit_loss
 from
  ( select  cs_catalog_page_sk as page_sk,
            cs_sold_date_sk  as date_sk,
            cs_ext_sales_price as sales_price,
            cs_net_profit as profit,
            cast(0 as decimal(7,2)) as return_amt,
            cast(0 as decimal(7,2)) as net_loss
    from catalog_sales
    union all
    select cr_catalog_page_sk as page_sk,
           cr_returned_date_sk as date_sk,
           cast(0 as decimal(7,2)) as sales_price,
           cast(0 as decimal(7,2)) as profit,
           cr_return_amount as return_amt,
           cr_net_loss as net_loss
    from catalog_returns
   ) salesreturns,
     date_dim,
     catalog_page
 where date_sk = d_date_sk
       and d_date between cast('2000-08-23' as date)
                  and date_add(cast('2000-08-23' as date),  14)
       and page_sk = cp_catalog_page_sk
 group by cp_catalog_page_id)
 ,
 wsr as
 (select web_site_id,
        sum(sales_price) as sales,
        sum(profit) as profit,
        sum(return_amt) as returns,
        sum(net_loss) as profit_loss
 from
  ( select  ws_web_site_sk as wsr_web_site_sk,
            ws_sold_date_sk  as date_sk,
            ws_ext_sales_price as sales_price,
            ws_net_profit as profit,
            cast(0 as decimal(7,2)) as return_amt,
            cast(0 as decimal(7,2)) as net_loss
    from web_sales
    union all
    select ws_web_site_sk as wsr_web_site_sk,
           wr_returned_date_sk as date_sk,
           cast(0 as decimal(7,2)) as sales_price,
           cast(0 as decimal(7,2)) as profit,
           wr_return_amt as return_amt,
           wr_net_loss as net_loss
    from web_returns left outer join web_sales on
         ( wr_item_sk = ws_item_sk
           and wr_order_number = ws_order_number)
   ) salesreturns,
     date_dim,
     web_site
 where date_sk = d_date_sk
       and d_date between cast('2000-08-23' as date)
                  and date_add(cast('2000-08-23' as date),  14)
       and wsr_web_site_sk = web_site_sk
 group by web_site_id)
  select  channel
        , id
        , sum(sales) as sales
        , sum(returns) as returns
        , sum(profit) as profit
 from 
 (select 'store channel' as channel
        , 'store' || s_store_id as id
        , sales
        , returns
        , (profit - profit_loss) as profit
 from   ssr
 union all
 select 'catalog channel' as channel
        , 'catalog_page' || cp_catalog_page_id as id
        , sales
        , returns
        , (profit - profit_loss) as profit
 from  csr
 union all
 select 'web channel' as channel
        , 'web_site' || web_site_id as id
        , sales
        , returns
        , (profit - profit_loss) as profit
 from   wsr
 ) x
 group by rollup (channel, id)
 order by channel
         ,id
 limit 100;

Measured execution times:
4000MB: 2529
4000MB-sp(lt0.2/ts5/min10/max100): 2536
8000MB: 4404
8000MB-sp(lt0.2/ts5/min10/max100): 2576