On March 4, 2014 8:39:55 PM CET, Simon Riggs <simon@2ndQuadrant.com> wrote:
>On 4 March 2014 16:27, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Alvaro Herrera <alvherre@2ndquadrant.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...
Mvcc scans only guarantee that individual scans are consistent, not that separate scans are. Each individual scan takes
anew snapshot if there's been ddl.
Andres
--
Please excuse brevity and formatting - I am writing this on my mobile phone.
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
