Ah, I forgot CLUSTER was something run by hand on PG. Same footgun then, but you have to load and aim it yourself instead of being fully automatic like it is in MySQL, where it appears you can't opt out of clustering by the PK (similar story in SQL Server, but you can change which index it clusters by). Thanks for the clarification.

This is a bit pedantic, but you’re conflating MySQL with InnoDB. (In MySQL’s model, you can opt out of having a clustered index by choosing a storage engine that doesn’t use clustering.)

In practical terms, choosing a non-InnoDB storage engine is pretty rare with upstream MySQL, but perhaps slightly less so in Percona Server or MariaDB.