I'm not that knowledgeable about database stuff, though I did, in a previous life, work as a contractor at one time or another for Oracle, Informix, and Sybase. (I did backend stuff, meat and potatoes C programming, not database stuff with SQL or anything like that.) I was there when Informix kind of bit the dust and developers were leaving in droves for Oracle. (Sigh, Informix was a good place to work too, but then they got a CEO who screwed things up. Enough digression.)
From what I've read and heard, open source Postgres is pretty good. It has not become the linux of database engines though, and, what I heard is, it's because it just can't do the heavy lifting of zillions of transactions that Oracle can. Still, I can't help but wonder. Maybe they need a Redhat for Postgres to beef it up and supply support for those who need hand holding. (I know, I know, Redhat! But they did some good stuff in the beginning. Maybe Redhat has gone over to the Dark Side, but modern Linux benefited from them, my opinion.)