MySQL folks take a lot of things for granted that are not present in this form in Postgres, and that may be a bad surprise when you try out "the other system".
MySQL for example has a stable and upwards- and slightly-downwards-compatible on-disk format, mostly. Oracle sometimes forgets how important that is and makes changes, or unannounced changes, and then needs to be corrected, hard, by their development partners and testers.
@isotopp I definitely prefer PostgreSQL to #MySQL, but #PostgreSQL deciding that collation derives from your system's locale settings when PostgreSQL installed means identical setups on different boxes can behave differently. This is NOT FUN to debug.
I've heard that there was discussion to change this, but I don't know if it happened.
On-disk database formats are always in-place upgrade-able in binary form. They are usually downgrade-able at least between the current and the previous form.
If not, this is documented.
Strategies exist to migrate a replication tree, live, and transparently to a newer or back to the previous version of the database.
Replication wire format is compatible across a large range of versions, and guaranteed to be so for version-forward trees (ie the primary is the oldest version of the database).
Replication events are materialized on disk in the binlog, and replication scales out cheaply, to hundreds of replicas from a single primary or intermediate.
Databases can be cloned by snapshotting and recovering the datadir, copying it over to a new instance and making the copy a replica of the primary. This is straightforward even without GTID, and automatic with GTID.
Tooling exists to automatically manage, reshape, grow and shrink replication trees based on the results of predictive autosizing information.
That means, DBAs can drag and drop nodes across a replication tree in a web interface and the replicas will be detached and reattached in the replication tree at will, including changes to the primary.
This makes several replication based HA concepts viable.
All of these things were done intentionally. I know this, because I had and continue to have firm opinions based on personal operational experience that I injected into these discussions at the appropriate time.
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