We used PostgreSQL at my old startup and switch away from MYSQL. Some of the things I liked about PostgreSQL; that you can do schema updates without rewriting and locking the table. The one thing we ran into, because of the way MVCC works, when you update a column the whole row gets rewritten. For example; if you have large rows and you update a Boolean column in a bunch of them - you will get a large write load. We ran on AWS and basically maxed out the PIOPs when updating a Boolean column on ~2m record table. Moreover, this would effect our read speeds causing our web site to be super slow. There was no easy way to throttle the MVCC writes.