The rank() and dense_rank() functions are, in my judgment, the most prominent functions in SQL. The rank() function returns the number of the current row within its window. Counting starts at 1.

Here is an example:

test=# SELECT year, production, 
              rank() OVER (ORDER BY production) 
FROM  t_oil 
WHERE country = 'Other Middle East' 
ORDER BY rank 
LIMIT 7; 
 year  | production | rank 
-------+------------+------ 
 2001  |  47        |  1 
 2004  |  48        |  2 
 2002  |  48        |  2 
 1999  |  48        |  2 
 2000  |  48        |  2 
 2003  |  48        |  2 
 1998  |  49        |  7  
(7 rows) 

The rank column will number those tuples in your dataset. Note that many rows in my sample are equal. Therefore, the rank will jump from 2 to 7 directly, because many production values are identical. If you want to avoid that, the dense_rank() function is the way to go about this:

test=# SELECT year, production, 
              dense_rank() OVER (ORDER BY production) 
FROM  t_oil 
WHERE country = 'Other Middle East' 
ORDER BY dense_rank 
LIMIT 7; 
 year  | production | dense_rank 
-------+------------+------------ 
 2001  |         47 |          1 
 2004  |         48 |          2 
 ... 
 2003  |         48 |          2 
 1998  |         49 |          3  
(7 rows)  

PostgreSQL will pack the numbers more tightly. There will be no more gaps.