> I wonder why/how MySQL got so popular, when in my head the go-to is Postgres (perhaps because I worked on Django/Rails stuff early on). Is it because of Oracle and their marketing/sales work?

MySQL's popularity long predates the Oracle acquisition. MySQL got popular because it was usually available (alongside PHP) on shared hosting and had very fast read performances (using the default MyISAM storage, at the cost of write performances or actually caring about your data). What with the web being front-loaded on read, that made it a pretty good fit.

Meanwhile historically postgres was byzantine to set up and slow. It cared much more for your data and had plenty of features (custom types! procedural languages! transactional DDL!), but it didn't really work OOTB (still doesn't, IIRC the base configuration still assumes rotating rust and 128MB RAM or some shit) and wasn't available on most hosting platforms, especially not the really cheap shared hosts.

Also while Django defaulted to postgres, I'm pretty sure Rails used to kinda-sorta default to mysql (I'm talking 10 years back, many plugins/extensions didn't work properly on pg).