Re: ALTER TABLE lock strength reduction patch is unsafe

Noah Misch <noah(at)leadboat(dot)com> writes:
> Just to be clear, that list is not a commentary on the particular patch at
> hand. Those are merely the kinds of regressions to look for in a patch
> affecting this area of the code.

A complaint on pgsql-bugs just now reminded me of a specific area that
needs to be looked at hard: how bad are the implications for pg_dump?

Up to now, pg_dump could be reasonably confident that once it had
AccessShareLock on every table it intended to dump, there would be no
schema changes happening on those tables until it got done. This greatly
ameliorates the snapshot-skew problems that arise from its habit of doing
some things for itself and other things via backend-internal functions
(which historically used SnapshotNow and now use a fresh MVCC snapshot,
either way potentially quite newer than the transaction snapshot pg_dump's
own queries will use).

I suspect that lowering the lock requirements for anything that affects
what pg_dump can see is going to make things a whole lot worse in terms of
consistency of pg_dump output in the face of concurrent DDL. Admittedly,
we're not perfect in that area now, but I'm not sure that's an adequate
excuse for making it worse.

regards, tom lane