How to update a datagrid column separately after page is loaded?

[WizPS]

For one case I use a data table where one column is a count of existent rows in another table for that row combination. A normal joint view with a count works but in my case the second table contains millions rows which slows the view or query down. So I would like the original table to load fully and then that count column to be separately loaded with a spinning wheel while loading or similar. Only the count of rows for the ones being displayed in the datagrid needs to be extracted. If the user reloads by pagination or filer or so, that set needs to be post loaded again. Anyone done something similar? Any ideas of how to achieve this?