Re: Documentation fixes for pg_visibility

On Wed, Jun 29, 2016 at 1:42 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> 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.

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?

--
Robert Haas
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