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.

Attachment: vm-all-frozen-check.patch
Description: invalid/octet-stream

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