Re: ALTER TABLE lock strength reduction patch is unsafe

On Fri, Jun 17, 2011 at 3:45 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> Department of second thoughts: I think I see a problem.
>
> Um, yeah, so that doesn't really work any better than my idea.
>
> On further reflection, there's a problem at a higher level than this
> anyway.  Even if we can get a single SnapshotNow scan to produce
> guaranteed-self-consistent results, that doesn't ensure consistency
> between the results of scans occurring serially.  An example here is
> ALTER COLUMN DROP DEFAULT, which is currently imagined to impact only
> writers.  However, suppose that a concurrent relcache load fetches the
> pg_attribute row, notes that it has atthasdef = true, and then the ALTER
> commits before we start to scan pg_attrdef.  The consistency checks in
> AttrDefaultFetch() will complain about a missing pg_attrdef entry, and
> rightly so.  We could lobotomize those checks, but it doesn't feel right
> to do so; and anyway there may be other cases that are harder to kluge up.
>
> So really we need consistency across *at least* one entire relcache load
> cycle.  We could maybe arrange to take an MVCC snap (or some lighter
> weight version of that) at the start, and use that for all the resulting
> scans, but I think that would be notationally messy.  It's not clear
> that it'd solve everything anyhow.  There are parts of a relcache entry
> that we fetch only on-demand, so they are typically loaded later than
> the core items, and probably couldn't use the same snapshot.  Worse,
> there are lots of places where we assume that use of catcache entries or
> direct examination of the catalogs will yield results consistent with
> the relcache.
>
> I suspect these latter problems will impact Simon's idea as well.

I suspect we're going to be told that they don't.

I suspect I'm not going to believe it.

--
Robert Haas
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