On Fri, Jul 1, 2016 at 6:58 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Jun 29, 2016 at 1:42 AM, Michael Paquier > <michael.paqu...@gmail.com> wrote: >> On Tue, Jun 28, 2016 at 7:05 AM, Robert Haas <robertmh...@gmail.com> wrote: >>> On Mon, Jun 27, 2016 at 5:56 PM, Michael Paquier >>> <michael.paqu...@gmail.com> wrote: >>>>> Under what circumstances would you wish to check only one page of a >>>>> relation? >>>> >>>> What I'd like to be able to do is to stop scanning the relation once >>>> one defective tuple has been found: if there is at least one problem, >>>> the whole vm needs to be rebuilt anyway. So this function could be >>>> wrapped in a plpgsql function for example. It is more flexible than >>>> directly modifying this function so as it stops at the first problem >>>> stopped. >>> >>> I think most likely the best way to handle this is teach VACUUM to do >>> PageClearAllVisible() and visibilitymap_clear() on any page where >>> VM_ALL_FROZEN(onerel, blkno, &vmbuffer) && !all_frozen. This would go >>> well with the existing code to clear incorrectly-set visibility map >>> bits, and it would allow VACUUM (DISABLE_PAGE_SKIPPING) to serve the >>> purpose you're talking about here, but more efficiently. >> >> Ah, I see. So your suggestion is to do this job in lazy_scan_heap() >> when scanning each block, and then to issue a WARNING and clear the >> visibility map. Indeed that's better. I guess I need to take a closer >> look at vacuumlazy.c. See attached for example, but that's perhaps not >> something to have in 9.6 as that's more a micro-optimization than >> anything else. > > Right, something like that. I think Andres actually wants something > like this in 9.6, and I'm inclined to think it might be a good idea, > too. I think there should probably be a test for > all_visible_according_to_vm at the beginning of that so that we don't > add more visibility map checks for pages where we already know the VM > bit can't possibly be set. > > Other opinions on the concept or the patch? >
+1 on the idea. + PageClearAllVisible(page); + MarkBufferDirty(buf); What is the need to clear the Page level bit, if it is already cleared, doesn't '!all_frozen' indicate that? -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
