On Thu, Jul 3, 2014 at 3:38 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> 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.)
Yes, that's what the current code does btw, in this *messy* way.

>> 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.
Yeah thought so.

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