Wikipedia is some MySQL databases with read-only slaves front-ended by Ngnix caches and load balancers. That seems to get the job done. Wikipedia is the fifth busiest web site in the world.
Netflix's web site (not the playout system) was originally a bunch of Python programs.
The article mentions a PostGres query that required a full table scan. If you're doing many queries that require a full table scan, you're doing something wrong. That's what indices are for.