[jasonhansel]

It's relational, it scales horizontally, and it's free. That combination is surprisingly hard to find.

[zem]

i'm not a database person, but does postgres not scale horizontally?

[kasey_junk]

No. Read replication approaches linear horizontal scaling (though global latencies can lead to weird edge cases).

Write replication requires external products.

[jasonhansel]

Specifically, Postgres extensions that add write sharding are generally either (a) non-FOSS commercial products or (b) not very well maintained.