Before 8.0, a DROP TABLE naming three tables that failed on the second one had already dropped the first, and a crash during an ALTER could leave an orphaned temporary table on disk.
-- 5.7: drops t1, fails on t2, leaves t3 alone. no rollback.
-- 8.0: all three, or none.
DROP TABLE t1, t2, t3;
-- and the artefacts that used to survive a crash
SELECT * FROM information_schema.innodb_tables WHERE name LIKE '%#sql%';The change is that data dictionary updates are now in an InnoDB transaction rather than in files the server rewrites separately. It does not make DDL transactional in the sense of being rollback-able from application code — you still cannot wrap an ALTER in a transaction with inserts and undo the lot. What it removes is the recovery work after a failed or interrupted schema change, which was a real category of two-in-the-morning problem.