> Adding a field in postgres while setting a default value would be exactly the same as adding a field Not true. Postgres locks the entire table for the durati

> Adding a field in postgres while setting a default value would be exactly the same as adding a field Not true. Postgres locks the entire table for the duration of the update (which could be hours or days for a large enough dataset). Mongo will lock your entire databased for tiny fractions of a second, lots and lots of times. The difference is huge. The postgres implementation takes down your site, the Mongo implementation doesn't. At scale, people start using Postgres a lot like a NoSQL service. Check out the Reddit schema for example, or read IMVU's blog posts on the topic (with MySQL, but same point). Or Facebook's architecture. All those migrations strategies look a lot more like mongo than postgres, even though they all use SQL DBs. > Now I'm not anti-MongoDB, I'm just saying you shouldn't give it credit for something that relational database do just fine. Saying "I can do NoSQL migrations just fine in Postgres" is like saying "I can do OO just fine in assembly".