Postgres comes with the building blocks for both sharding and HA out of the box, and they're extensively discussed in the docs. You don't need proprietary addons other than as pure convenience.
don't underestimate the importance of convenience. I'm convinced one of the reasons MySQL had so much more mindshare than postgres back in the day was that it was far easier to get up and running, even if postgres might have been easier to use once everything was set up correctly.
I've been using Postgres since 1998, and I tried getting MySQL up first. There was more documentation available for the latter, so it should have been simple. Failed. It just didn't work.
Out of frustration I then tried Postgres, because I just wanted a decent database for my project. It was surprisingly easy, I only had to learn about pg_hba.conf to get to a functional state. Everything else was in place out of the box.
I've been a happy user ever since. MySQL may have had the mindshare (thanks to prevalence of LAMP) but everything outside the magic happy path was confusing and fragile.
No one cares what was in 1998, that's the whole point of this discussion. Postgres devs kept digging their heads into sand for many years, saying that high availability is somehow not the task of the database. In reality only the datastores need to be HA/durable in an otherwise stateless architecture.
It still has some features over PostgreSQL that pushed me to choose it (actually MariaDB) for a new project about a year ago, namely multi-master replication. Yes, I know, terrible database and horrible feature, but it really helped in that particular domain.
I couldn't find anything decent for postgres, while MariaDB/MySQL have that built-in, with some differences in implementation. Especially for a customer who refuses to pay for his software, because there are some commercial solutions.
