Michael Paquier <michael.paqu...@gmail.com> writes:
> On Wed, Jul 2, 2014 at 5:32 PM, Kyotaro HORIGUCHI
> <horiguchi.kyot...@lab.ntt.co.jp> wrote:
>> Pehaps this is showing that no tidy or generalized way to tell
>> what a page is used for. Many of the modules which have their
>> own page format has a magic value and the values seem to be
>> selected carefully. But no one-for-all method to retrieve that.

> You have a point here.

Yeah, it's a bit messy, but I believe it's currently always possible to
tell which access method a PG page belongs to.  Look at pg_filedump.
The last couple times we added index access methods, we took pains to
make sure pg_filedump could figure out what their pages were.  (IIRC,
it's a combination of the special-space size and contents, but I'm too
tired to go check the details right now.)

> For gin, I'll investigate if it is possible to add a identifier like
> GIN_PAGE_ID, it would make the page analysis more consistent with the
> others. I am not sure for what the 8 bytes allocated for the special
> area are used now for though.

There is exactly zero chance that anyone will accept an on-disk format
change just to make this prettier.

                        regards, tom lane

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