Glossary
Indexed column
An indexed column is a SharePoint list column marked so that filtering and sorting on it stays fast and works past the 5,000 item view threshold. Adding one is a list setting, not a migration.
It is the answer to almost every “our list is too big” complaint. Index the columns your views filter on, make the views filter rather than return everything, and the problem usually disappears.
Worth doing before the list grows: indexing an already-large list can itself run into the threshold, which is a mildly annoying chicken-and-egg situation.