I have done some research and I think I am ok, however I cannot take the risk of not knowing for sure. I have just developed a site that will host 8000 events per year (8000 List items) and there will be around 120,000 members (120,000 List items) at those events.
I have set up my regional column (single choice) to be indexed and I have a query string filter configured to filter based on the region you choose. I have found plenty on ensuring you have a filter on an indexed column, however they are always using basic list filters in the examples. Will a query string filter work to keep my events list under the list view threshold?
Second part for the members, no event will ever hold more than 1000 people in it. I have another query string filter set up to filter the member's list based on the event-ID. Will the query string filter keep me under the threshold? I think I will index the Event-ID column as this is the unique key that brings this entire system together.