How to tell if a query will scale well?

Question

What are some of the methods/techniques experienced SQL developers use to determine if a particular SQL query will scale well as load increases, rows in associated tables increa

Answer 1

don't make tables wide, keep them narrow as well as the indexes. Make sure that queries are fully covered by indexes and that those queries are SARGable.

Test with a ton of data before going in production, take a look at this: Your testbed has to have the same volume of data as on production in order to simulate normal usage