Re: gist vacuum gist access

On 09/08/2014 11:19 AM, Alexander Korotkov wrote:
> On Mon, Sep 8, 2014 at 12:08 PM, Alexander Korotkov <aekorotkov(at)gmail(dot)com>
> wrote:
>
>> On Mon, Sep 8, 2014 at 11:13 AM, Heikki Linnakangas <
>> hlinnakangas(at)vmware(dot)com> wrote:
>>
>>> In the b-tree code, we solved that problem back in 2006, so it can be
>>> done but requires a bit more code. In b-tree, we solved it with a "vacuum
>>> cycle ID" number that's set on the page halves when a page is split. That
>>> allows VACUUM to identify pages that have been split concurrently sees
>>> them, and "jump back" to vacuum them too. See commit
>>> http://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=
>>> 5749f6ef0cc1c67ef9c9ad2108b3d97b82555c80. It should be possible to do
>>> something similar in GiST, and in fact you might be able to reuse the NSN
>>> field that's already set on the page halves on split, instead of adding a
>>> new "vacuum cycle ID".
>> ...
>
> Another note. Assuming we have NSN which can play the role of "vacuum cycle
> ID", can we implement sequential (with possible "jump back") index scan for
> GiST?

Yeah, I think it would work. It's pretty straightforward, the page split
code already sets the NSN, just when we need it. Vacuum needs to
memorize the current NSN when it begins, and whenever it sees a page
with a higher NSN (or the FOLLOW_RIGHT flag is set), follow the
right-link if it points to lower-numbered page.

- Heikki