Well, I believe you could use the streaming algorithms to pick the likely median, so help choose the pivot for the real quickselect. quickselect can be done inplace too which is O(1) memory if you can afford to rearrange the data.
say you want the median value since the beginning of the day for a time series that has 1000 entries per second, that's potentially hundreds of gigabytes in RAM, or just a few variables if you use a p-square estimator.
