The largest ad system ever, Google AdSense, used MySQL until circa 2014 when it moved to a completely custom DB backend, F1. F1 is also a SQL database, however.
Google does use bigtable and such where appropriate, but for anything more complex than dumb key/value you can't do much better than regular DBs. Some people think they can, but 99% of the time they're mistaken.
Google does use bigtable and such where appropriate, but for anything more complex than dumb key/value you can't do much better than regular DBs. Some people think they can, but 99% of the time they're mistaken.