The performance impact is at write time, and having the data cached in memory is irrelevant. It's the locking overhead that's non-trivial, especially if each ta

The performance impact is at write time, and having the data cached in memory is irrelevant. It's the locking overhead that's non-trivial, especially if each table has several FKs, when operating in a high-volume OLTP environment. As for "database engines that properly support online schema changes without locking, downtime or table renaming hacks", please name some. In my experience, every major DBMS has cases where certain ALTERs block concurrent DML, which is extremely problematic on very large tables. Or even cases where there's no locking, but the operation still takes a very long time, which is conceptually problematic for logical replication.