I'm pretty perplexed as well. They mention that they're not sharding PostgreSQL, without mentioning why in the article, but isn't that an obvious issue to many of their scaling problems?

I don't really see what it is that they're doing that requires a single master database, it seems that sharding on a per user basis would make things way easier for them.

I can sympathise with wanting to stay on a single master architecture as long as possible. Sharding is a major step change in terms of maintenance burden.

My guess is that they didn’t design their schema around multi tenancy, and going back to pick apart all the shared tables, rewrite queries etc… will take them more time than they want to spend.