Understanding hypothetical aggregates

Hypothetical aggregates are pretty similar to standard ordered sets. However, they help answer a different kind of question: what would be the result if a value was in the data? As you can see, this is not about values inside the database, but about the result if a certain value was actually there.

The only hypothetical function that's provided by PostgreSQL is rank:

test=# SELECT region, 
              rank(9000) WITHIN GROUP 
                         (ORDER BY production DESC NULLS LAST) 
FROM t_oil 
GROUP BY ROLLUP (1); 
    region     | rank 
---------------+------ 
 Middle East   |  21 
 North America |  27 
               |  47 
(3 rows)

It tells us this: if somebody produced 9,000 barrels per day, it would be ranked the 27th best year in North America and 21st in the Middle East.