SELECT t.year, p.category, SUM(ps.purchase_price) as sum_of_sales
FROM time t, product p, purchases ps
WHERE t.time_key = ps.time_key AND
      ps.product_id = p.product_id AND
      t.month_name = 'January'

GROUP BY t.year, p.category;

  Id    Operation                       Name              
__________________________________________________
   0    SELECT STATEMENT                                 
   1     SORT GROUP BY                                    

   2      HASH JOIN                                       
   3       HASH JOIN                                      
   4        VIEW                                          
   5         SORT UNIQUE                                  
   6         TABLE ACCESS FULL          TIME             
   7       MAT_VIEW REWRITE ACCESS FULL  MONTHLY_SALES_MV 
   8      TABLE ACCESS FULL             PRODUCT