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From: Heikki Linnakangas on 23 Jun 2010 11:33
On 23/06/10 18:17, Leonardo F wrote:
> in bufpage.h:
>
> "all blocks written out by an access method must be disk pages"
>
> but in
> http://www.postgresql.org/docs/8.4/static/storage-page-layout.html
>
> "Actually, index access methods need not use this page format. All the
> existing index methods do use this basic format, but the data kept on
> index metapages usually doesn't follow the item layout rules."
>
> I'm not getting it: am I supposed to use the "disk page format" when
> writing an index access method, or it's just a "good practice" because
> it makes the handling easier? Given the docs it looks "recommended",
> but the comment on the code sounds more "mandatory".

It's recommened because it makes life easier. Even if you don't use the
normal page format, I think you need to have the LSN at the beginning of
the page. The rest is up to you.

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

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From: Tom Lane on 23 Jun 2010 14:24
Heikki Linnakangas <heikki.linnakangas(a)enterprisedb.com> writes:
> On 23/06/10 18:17, Leonardo F wrote:
>> I'm not getting it: am I supposed to use the "disk page format" when
>> writing an index access method, or it's just a "good practice" because
>> it makes the handling easier? Given the docs it looks "recommended",
>> but the comment on the code sounds more "mandatory".

> It's recommened because it makes life easier. Even if you don't use the
> normal page format, I think you need to have the LSN at the beginning of
> the page. The rest is up to you.

The bufpage.h comment is ancient and might not be 100% gospel anymore.
However, I believe the buffer manager does assume that any page in
shared buffers has a standard page header including LSN. It definitely
assumes there's an LSN, and I think we have validity checks on the other
page header fields somewhere inside ReadBuffer. You don't however need
to use the item-and-item-pointer-array structure if it's not useful to
you.

regards, tom lane

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