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SQL> EXPLAIN PLAN FOR
 2   select prod_category, avg(amount_sold)
 3   from sales s, products p
 4   where p.prod_id = s.prod_id
 5   group by prod_category;

Explained.

SQL>  select plan_table_output
from table(dbms_xplan.display('plan_table',null,'basic +predicate +cost'));

------------------------------------------
 Id   Operation              Name   
------------------------------------------
   0  SELECT STATEMENT              
   1   HASH GROUP BY                
   2    HASH JOIN                   
   3     TABLE ACCESS FULL   PRODUCTS
   4     PARTITION RANGE ALL        
   5      TABLE ACCESS FULL  SALES  
------------------------------------------