On Wed, Jun 15, 2016 at 2:45 PM, Alvaro Herrera <alvhe...@2ndquadrant.com> wrote: > Robert Haas wrote: >> On Wed, Jun 15, 2016 at 1:44 PM, Kevin Grittner <kgri...@gmail.com> >> wrote: >> > The test I showed creates a situation which (to ANALYZE) is >> > identical to what you describe -- ANALYZE sees a page with an LSN >> > recent enough that it could have been (and actually has been) >> > pruned. Why would it be better for the ANALYZE to fail than to >> > complete? >> >> As I understand it, the reason we need to sometimes give "ERROR: >> snapshot too old" after early pruning is because we might otherwise >> give the wrong answer. > > So what constitutes "the wrong answer"? A regular transaction reading a > page and not finding a tuple that should have been there but was > removed, is a serious problem and should be aborted. For ANALYZE it may > not matter a great deal. Some very old tuple that might have been > chosen for the sample is not there; a different tuple is chosen instead, > so the stats might be slightly difference. No big deal. > > Maybe it is possible to get into trouble if you're taking a sample for > an expression index.
The expression index case is the one to worry about; if there is a problem, that's where it is. What bothers me is that a function used in an expression index could do anything at all - it can read any table in the database. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
