median and percentile function implementation

Hello

Three years ago we worked on implementation of median function.

http://www.postgresql.org/message-id/AANLkTimRksUOgGsK7-gTXnvJJGL-1QvuqxidusZQwep6@mail.gmail.com

Can we implement this function now?

There are a few opened questions:

* immutable parameters of aggregate functions

* effective implementation when data are in memory

* effective implementation when data are out of memory

* effective implementation for usage as window function - running median.

Better than before, we can choose between hashing and sorting based
aggregation. But tuplesort implementation is not right for using as window
function. Second issue can be a not possible reusing of tuplesort for more
final functions - median, percentile in one query.

Ideas how to solve this task? Can we move forward in some points?

Regards

Pavel