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. -- Michael
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