Re: ALTER TABLE lock strength reduction patch is unsafe

On 4 March 2014 16:27, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
>> One concern is schema changes that make a dump unrestorable, for
>> instance if there's a foreign key relationship between tables A and B,
>
> Yeah. Ideally, what pg_dump would produce would be a consistent snapshot
> of the database state as of its transaction snapshot time. We have always
> had that guarantee so far as user data was concerned, but it's been shaky
> (and getting worse) so far as the database schema is concerned. What
> bothers me about the current patch is that it's going to make it a whole
> lot more worse.

While thinking this through it occurs to me that there is no problem at all.

Your earlier claim that the dump is inconsistent just isn't accurate.
We now have MVCC catalogs, so any dump is going to see a perfectly
consistent set of data plus DDL. OK the catalogs may change AFTER the
snapshot was taken for the dump, but then so can the data change -
that's just MVCC.

I was going to add an option to increase lock level, but I can't see
why you'd want it even. The dumps are consistent...

--
Simon Riggs http://www.2ndQuadrant.com/
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