Re: Hot standby and GiST page splits (was Re: WIP: Fast GiST index build)

On 01.08.2011 13:13, Simon Riggs wrote:
> On Mon, Aug 1, 2011 at 10:38 AM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>> Attached is a patch for that for 9.1/master. The 9.0 GiST replay code was
>> quite different, it will require a separate patch.
>
> Hmm, I was assured no changes would be required for Hot Standby for
> GIST and GIN. Perhaps we should check GIN code also.

Yeah, we probably should.

> Does the order of locking of the buffers matter? I'm sure it does.

Yep.

> Did you want me to write the patch for 9.0?

I'm looking at it now.

> And what does NSN stand for? :-)

Hmm, I don't know actually know what NSN is an acronym for :-). But in
case you want an explanation of what it does:

The NSN is field in the GiST page header, used to detect concurrent page
splits. Whenever a page is split, its NSN is set to the LSN of the page
split record. To be precise: the NSN of the resulting left page(s) is
set, the resulting rightmost half keeps the NSN of the original page.

When you scan a parent page and decide to move down, it's possible that
the child page is split before you read it, but after you read the
parent page. So you didn't see the downlink for the right half when you
scanned the parent page. To reach the right half, you need to follow the
rightlink from the left page, but first you need to detect that the page
was split. To do that, when you scan the parent page you remember the
LSN on the parent. When you scan the child, you compare the parent's LSN
you saw with the NSN of the child. If the child's NSN > parent's LSN,
the page was split after you scanned the parent, so you need to follow
the rightlink.

The B-tree code has similar move-right logic, but it uses the "high" key
on each page to decide when it needs to move right. There's no high key
on GiST pages, so we rely on the NSN for that.

In 9.1, I added the F_FOLLOW_RIGHT flag to handle the case of an
incomplete split correctly. If the flag is set on a page, its rightlink
needs to be followed regardless of the NSN.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com