While everybody is going to be rightfully excited about the logical replication, for me personally, CREATE STATISTICS and the new ROW syntax for UPDATE amount to the additions that have the probably biggest effect on me ever since I moved to postgres exclusively when 7.1 was released. Especially CREATE STATISTICS (wonderful explanation here https://www.postgresql.org/docs/10.0/static/multivariate-sta... ) is the one…
As exciting as this is (and it really is) it's yet another thing that explodes the number of possibilities for optimization. It's getting towards the point of unmanageability and I hope there will be a movement towards, if not auto-tuning databases, assistive tools for exploring both this and indexing possibilities.
The ultimate goal is to allow detecting this automatically, and either create the statistics right away or at least providing some suggestions to the users. That's not in Pg10 though.