Hypothetical aggregates are pretty similar to standard ordered sets. However, they help to answer a different kind of question: what would be the result if a value was there? 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 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: If somebody produced, 9000 barrels per day, it would be ranked the 27th best year in North America and 21st in the Middle East.
