Actually both lock for many (crucial) schema operators, and often severely enough to block your application from reading from the table(s) under change. I've been researching this stuff for a while. Check out http://blog.minicom.nl/blog/2015/04/03/revisiting-profiling-... . It's slightly outdated, but still holds.

That is true, but I wanted to share another angle that may or may not affect PostgreSQL while it continues to affect MySQL even as it has crash-safe (though not transactional) DDL now: these schema changes are online for the master, but are not replication-aware and can have impact in replication delay on servers down the hierarchy.

For this reason alone I think we'll continue to use schema-change tools on MySQL even if the server itself becomes better at those.

In the specific case of gh-ost, another good point is that migrations can be completely paused, which in MySQL is not true of online DDL.