Historically, native online DDL in MySQL 5.6+ / MariaDB 10+ isn't replication-friendly -- despite being non-blocking on the primary, it blocks the replication s

Historically, native online DDL in MySQL 5.6+ / MariaDB 10+ isn't replication-friendly -- despite being non-blocking on the primary, it blocks the replication stream application on replicas, which makes it basically unusable at scale. (This is inherently a trade-off of MySQL/MariaDB using logical replication, instead of physical replication of the tx log / WAL.) The newer INSTANT algo in MySQL 8 and MariaDB 10.3+ solves this, but it is only usable for a limited subset of alter operations, such as adding a new column. That's one of the most common ALTER cases, so this feature is quite nice, but it certainly doesn't solve everything. For this reason, external tools such as pt-online-schema-change are still pretty essential for MySQL/MariaDB deployments of any non-trivial size. MariaDB 10.8, which is still pre-GA, adds a clever solution to the replication problem: https://jira.mariadb.org/browse/MDEV-11675 https://jira.mariadb.org/browse/MDEV-11675 . It will be interesting to see if there are any real-world operational drawbacks to this approach, and seeing if MySQL offers this soon as well.