> ...an unexpected 1000x increase in volume at a customer brought the DB to its knees.
From an outside perspective it seems like the huge increase in volume was more the issue! It sounds like an index helped a lot, but it would also have added cost for all those customers who didn't see that jump in volume.
Well, of course the volume had something to do with it, but adding the missing index meant the system could easily handle that volume.
The other customers pay for that index as well of course but either the volume is low enough that it's trivial or it's large enough that they too saw ann increase in speed.
From an outside perspective it seems like the huge increase in volume was more the issue! It sounds like an index helped a lot, but it would also have added cost for all those customers who didn't see that jump in volume.