On Tue, Jun 28, 2016 at 7:05 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Mon, Jun 27, 2016 at 5:56 PM, Michael Paquier
> <michael(dot)paquier(at)gmail(dot)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