Performance of "distinct with limit"

Hi,

Ran into this under-optimized query execution.

select distinct n from bigtable;   -- Lets say this takes 2 minutes

select distinct n from bigtable limit 2  -- This takes approximately the same time

However, the latter should have the potential to be so much quicker.  I checked the same query on MSSQL (with 'top 2'), and it seems to do exactly the optimization I would expect. 

Is there any way to achieve a similar speedup in Postgresql?

Klaudie