Oracle 7.3 supports it!
That is how this all started. A gentleman tweeted about a Postgres limitation that Oracle has not had since at least since Oracle 7.3.
The problem
As you can see in the tweet, Postgres by default will not defer a PRIMARY KEY check. Without the check being deferred the following will not work:
postgres=# select * from demo; id ---- 1 2 (2 rows) postgres=# …