APPROXIMATE PERCENTILE_DISC function
APPROXIMATE PERCENTILE_DISC is an inverse distribution function that assumes a discrete distribution model. It takes a percentile value and a sort specification and returns an element from the given set. Approximation enables the function to run much faster, with a low relative error of around 0.5 percent.
For a given percentile value, APPROXIMATE PERCENTILE_DISC uses a quantile summary algorithm to approximate the discrete percentile of the expression in the ORDER BY clause. APPROXIMATE PERCENTILE_DISC returns the value with the smallest cumulative distribution value (with respect to the same sort specification) that is greater than or equal to percentile.
APPROXIMATE PERCENTILE_DISC is a computenode only function. The function returns an error if the query doesn't reference a userdefined table or Amazon Redshift system table.
Syntax
APPROXIMATE PERCENTILE_DISC ( percentile ) WITHIN GROUP (ORDER BY expr)
Arguments
 percentile

Numeric constant between 0 and 1. Nulls are ignored in the calculation.
 WITHIN GROUP ( ORDER BY expr)

Clause that specifies numeric or date/time values to sort and compute the percentile over.
Returns
The same data type as the ORDER BY expression in the WITHIN GROUP clause.
Usage notes
If the APPROXIMATE PERCENTILE_DISC statement includes a GROUP BY clause, the result set is limited. The limit varies based on node type and the number of nodes. If the limit is exceeded, the function fails and returns the following error.
GROUP BY limit for approximate percentile_disc exceeded.
If you need to evaluate more groups than the limit permits, consider using PERCENTILE_CONT function.
Examples
The following example returns the number of sales, total sales, and fiftieth percentile value for the top 10 dates.
select top 10 date.caldate, count(totalprice), sum(totalprice), approximate percentile_disc(0.5) within group (order by totalprice) from listing join date on listing.dateid = date.dateid group by date.caldate order by 3 desc; caldate  count  sum  percentile_disc +++ 20080107  658  2081400.00  2020.00 20080102  614  2064840.00  2178.00 20080722  593  1994256.00  2214.00 20080126  595  1993188.00  2272.00 20080224  655  1975345.00  2070.00 20080204  616  1972491.00  1995.00 20080214  628  1971759.00  2184.00 20080901  600  1944976.00  2100.00 20080729  597  1944488.00  2106.00 20080723  592  1943265.00  1974.00