nth percentile calculations in postgresql

With PostgreSQL 9.4 there is native support for percentiles now, implemented in Ordered-Set Aggregate Functions:

percentile_cont(fraction) WITHIN GROUP (ORDER BY sort_expression) 

continuous percentile: returns a value corresponding to the specified
fraction in the ordering, interpolating between adjacent input items
if needed

percentile_cont(fractions) WITHIN GROUP (ORDER BY sort_expression)

multiple continuous percentile: returns an array of results matching
the shape of the fractions parameter, with each non-null element
replaced by the value corresponding to that percentile

See the documentation for more details: http://www.postgresql.org/docs/current/static/functions-aggregate.html

and see here for some examples: https://github.com/michaelpq/michaelpq.github.io/blob/master/_posts/2014-02-27-postgres-9-4-feature-highlight-within-group.markdown

CREATE TABLE aa AS SELECT generate_series(1,20) AS a;
--SELECT 20

WITH subset AS (
    SELECT a AS val,
        ntile(4) OVER (ORDER BY a) AS tile
    FROM aa
)
SELECT tile, max(val)
FROM subset GROUP BY tile ORDER BY tile;

 tile | max
------+-----
    1 |   5
    2 |  10
    3 |  15
    4 |  20
(4 rows)