Re: Documentation of bt_page_items()'s ctid field
| От | Jeff Janes |
|---|---|
| Тема | Re: Documentation of bt_page_items()'s ctid field |
| Дата | |
| Msg-id | CAMkU=1w5YE8-n_QyLNPxnYuR7p-a-5+v_Y0p_CfDko+DTqs8bA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Documentation of bt_page_items()'s ctid field (Peter Geoghegan <pg@heroku.com>) |
| Ответы |
Re: Documentation of bt_page_items()'s ctid field
|
| Список | pgsql-hackers |
On Mon, Mar 9, 2015 at 4:06 PM, Peter Geoghegan <pg@heroku.com> wrote:
On Mon, Mar 9, 2015 at 3:51 PM, Jeff Janes <jeff.janes@gmail.com> wrote:
> How do I know if I am looking at a non-rightmost page?
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.
It will (as the
docpatch points out) also necessarily have a ""high key" item. We
know that we have to move right if the high key doesn't bound the
value we expected to find on the page (our scankey item - what the
index scan is searching for). So the high key goes with having a
rightlink, and in general we detect that we're looking at a
non-rightmost page based on the presence of a right-link.
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.
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.
Thanks,
Jeff
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