From: | Peter Geoghegan <pg(at)heroku(dot)com> |
---|---|
To: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
Cc: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Documentation of bt_page_items()'s ctid field |
Date: | 2015-03-10 00:34:52 |
Message-ID: | CAM3SWZQBqXzvmbvJnx36A6HRx1UnmK_kP+gVEvCgjMvYxEGjmQ@mail.gmail.com |
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Lists: | pgsql-hackers |
On Mon, Mar 9, 2015 at 5:18 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>> It has a right-link (that's the easiest way to tell).
>
>
> Meaning that btpo_next is not zero? Should we say that in the patch in so
> many words? I think it will be hard to explain the page_items more without
> also explaining the page_stats more.
Yes. And my wording above was sloppy: By definition, a non-rightmost
page is a page that has a rightlink (it will also have a highkey, but
that's certainly not how we distinguish them).
Attached patch adds a definition of non-rightmost in parenthesis,
which I think helps.
> So if I understand this correctly, if there is a high key it is itemoffset
> 1, but to know whether itemoffset 1 is a high key I first have to look at
> btpo_next.
Yes. The logic for caring about "minus infinity" items within internal
pages is hardcoded into _bt_compare(), so it's kind of a low-level
detail, even by these standards...but it comes up with pageinspect,
since they consume an IndexTuple. Confusion ensues.
> And if there is a minus infinity, it will either be itemoffset 2 or 1,
> depending on whether there is a high key or not.
That's correct. So "data" can be "(has data), (has no data), (has
data), (has data) .... " on a given page, which could freak users out.
--
Peter Geoghegan
Attachment | Content-Type | Size |
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bt_page_items_rightmost.patch | text/x-patch | 1.2 KB |
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