We have a list with 25000 items. The items can be seen in a view where there are no grouping/filtering. But if any of the filtering/grouping is done, the view breaks. All I need to do is group the items based on a column(This column is indexed). We don't want to split the list or increase the threshold. Any thoughts how this can be done?
1) After filtering does the view exceed 5000 items?
You also need to make sure that the first column of the filter does not return more items than the List View Threshold, even if the final result of the filtered view returns less than the List View Threshold.
2) Are there any items in the recycle bin?
If you are filtering a list by an indexed field, check your recycle bin for deleted items. Items in the recycle bin aren't actually deleted from the backend database until they are removed from the recycle bin. If the total number of filtered items in the list and in the recycle bin is greater than the List Threshold limit, then you may get an incomplete set of results, or none at all.
Bear in mind that even IF you decide to increase the threshold, increasing threshold is not an option in SharePoint Online.