We will try think of a way to detect whether the query provider of the source IQueryable (DataServiceQueryProvider in this particular case) supports the Distinct operation. If it does not, we can return the distinct values which are currently in memory, i.e. if you have paging the distinct values of the current page which is on the client.
Until we find a solution, you can simply turn off ShowDistinctFilters
on the column, which should eliminate the distinct request to the server. Let us know whether this is a viable workaround for the time being.
By the way, if you have an external way of bringing distinct values to the client, you can attach to the DistinctValuesLoading
event of the grid and supply your own list by assigning it to e.ItemsSource thus bypassing our default implementation.
the Telerik team