Query Q75 (degree of parallelism: 2)

WITH all_sales AS (
 SELECT d_year
       ,i_brand_id
       ,i_class_id
       ,i_category_id
       ,i_manufact_id
       ,SUM(sales_cnt) AS sales_cnt
       ,SUM(sales_amt) AS sales_amt
 FROM (SELECT d_year
             ,i_brand_id
             ,i_class_id
             ,i_category_id
             ,i_manufact_id
             ,cs_quantity - COALESCE(cr_return_quantity,0) AS sales_cnt
             ,cs_ext_sales_price - COALESCE(cr_return_amount,0.0) AS sales_amt
       FROM catalog_sales JOIN item ON i_item_sk=cs_item_sk
                          JOIN date_dim ON d_date_sk=cs_sold_date_sk
                          LEFT JOIN catalog_returns ON (cs_order_number=cr_order_number 
                                                    AND cs_item_sk=cr_item_sk)
       WHERE i_category='Books'
       UNION
       SELECT d_year
             ,i_brand_id
             ,i_class_id
             ,i_category_id
             ,i_manufact_id
             ,ss_quantity - COALESCE(sr_return_quantity,0) AS sales_cnt
             ,ss_ext_sales_price - COALESCE(sr_return_amt,0.0) AS sales_amt
       FROM store_sales JOIN item ON i_item_sk=ss_item_sk
                        JOIN date_dim ON d_date_sk=ss_sold_date_sk
                        LEFT JOIN store_returns ON (ss_ticket_number=sr_ticket_number 
                                                AND ss_item_sk=sr_item_sk)
       WHERE i_category='Books'
       UNION
       SELECT d_year
             ,i_brand_id
             ,i_class_id
             ,i_category_id
             ,i_manufact_id
             ,ws_quantity - COALESCE(wr_return_quantity,0) AS sales_cnt
             ,ws_ext_sales_price - COALESCE(wr_return_amt,0.0) AS sales_amt
       FROM web_sales JOIN item ON i_item_sk=ws_item_sk
                      JOIN date_dim ON d_date_sk=ws_sold_date_sk
                      LEFT JOIN web_returns ON (ws_order_number=wr_order_number 
                                            AND ws_item_sk=wr_item_sk)
       WHERE i_category='Books') sales_detail
 GROUP BY d_year, i_brand_id, i_class_id, i_category_id, i_manufact_id)
 SELECT  prev_yr.d_year AS prev_year
                          ,curr_yr.d_year AS year
                          ,curr_yr.i_brand_id
                          ,curr_yr.i_class_id
                          ,curr_yr.i_category_id
                          ,curr_yr.i_manufact_id
                          ,prev_yr.sales_cnt AS prev_yr_cnt
                          ,curr_yr.sales_cnt AS curr_yr_cnt
                          ,curr_yr.sales_cnt-prev_yr.sales_cnt AS sales_cnt_diff
                          ,curr_yr.sales_amt-prev_yr.sales_amt AS sales_amt_diff
 FROM all_sales curr_yr, all_sales prev_yr
 WHERE curr_yr.i_brand_id=prev_yr.i_brand_id
   AND curr_yr.i_class_id=prev_yr.i_class_id
   AND curr_yr.i_category_id=prev_yr.i_category_id
   AND curr_yr.i_manufact_id=prev_yr.i_manufact_id
   AND curr_yr.d_year=2002
   AND prev_yr.d_year=2002-1
   AND CAST(curr_yr.sales_cnt AS DECIMAL(17,2))/CAST(prev_yr.sales_cnt AS DECIMAL(17,2))<0.9
 ORDER BY sales_cnt_diff,sales_amt_diff
 limit 100;

Measured execution times:
4000MB: 6776
4000MB-sp(lt0.2/ts5/min10/max100): 7078
8000MB: 6577
8000MB-sp(lt0.2/ts5/min10/max100): 6951