a slightly stale comment

While mucking around in src/backend/utils/time/tqual.c today, I noticed
the following comment attached to HeapTupleSatisfiesNow:

* mao says 17 march 1993: the tests in this routine are correct;
* if you think they're not, you're wrong, and you should think
* about it again. i know, it happened to me. we don't need to
* check commit time against the start time of this transaction
* because 2ph locking protects us from doing the wrong thing.
* if you mess around here, you'll break serializability. the only
* problem with this code is that it does the wrong thing for system
* catalog updates, because the catalogs aren't subject to 2ph, so
* the serializability guarantees we provide don't extend to xacts
* that do catalog accesses. this is unfortunate, but not critical.

Much as I hate to disturb a comment just before its 19th birthday, the
bit about two-phase locking and serializability hasn't been correct
since around 1999 (when MVCC was added). :-)

Dan

--
Dan R. K. Ports MIT CSAIL http://drkp.net/