Consider this grid as an example. In grids, should I show the first page when user sort the grid by a column? I think logically I should. But I am not sure from UX point of view.
Yes you should. Let us say the user is in third page and sorts by "last post", he expects to see the first sorted record right away. This can be achieved only by showing the first page.
Given your example, there's no gain for the user if you stay on the same page. The reason is that there is no logical connection of the new sorting parameter and the current page.
A user is very likely to click back to the first page if they discover that the sorting did not reset the paging. So my suggestion is:
- Show the first page after resorting
- Be sure that it's clear for the user that they're back on page one (e.g. by temporarily highlighting the current page)
I admit the user is very likely to go back to the first page - but is that a reasonable expectation that they should do themselves? It might feel jumpy. I don't like when interfaces do things without my command.