As for scaling horizontally at the db level that can be achieved with foreign data wrappers calling out, but isn't built in.
While Postgres doesn't rival oracle and ms SQL server in feature checkboxes it's a very solid DBMS with many advanced SQL features, and it's free and open source. You can do a lot without hitting the scaling problems.
https://wiki.postgresql.org/wiki/Hot_Standby
As for scaling horizontally at the db level that can be achieved with foreign data wrappers calling out, but isn't built in.
While Postgres doesn't rival oracle and ms SQL server in feature checkboxes it's a very solid DBMS with many advanced SQL features, and it's free and open source. You can do a lot without hitting the scaling problems.