On March 4, 2014 8:39:55 PM CET, Simon Riggs <si...@2ndquadrant.com> wrote: >On 4 March 2014 16:27, Tom Lane <t...@sss.pgh.pa.us> wrote: >> Alvaro Herrera <alvhe...@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 a new 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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
