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Re: crash-safe visibility map, take three

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2010-12-01 17:23:50
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Lists: pgsql-hackers
On Wed, Dec 1, 2010 at 11:40 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> On 01.12.2010 18:25, Robert Haas wrote:
>> I think we can improve this a bit further by also introducing a
>> HEAP_XMIN_FROZEN bit that we set in lieu of overwriting XMIN with
>> FrozenXID.  This allows us to freeze tuples aggressively - if we want
>> - without losing any forensic information.  We can then modify the
>> above algorithm slightly, so that when we observe that a page is all
>> visible, we not only set PD_ALL_VISIBLE on the page but also
>> HEAP_XMIN_FROZEN on each tuple.
> Hmm, actually, if we're willing to believe PD_ALL_VISIBLE in the page header
> over the xmin/xmax on the tuples, we could simply not bother doing
> anti-wraparound vacuums for pages that have the flag set. I'm not sure what
> changes that would require outside heapam.c, as we'd have to be careful to
> not trust the xmin/xmax if the flag was set.
> The first update on the page that clears the flag would need to freeze all
> the tuples in that scheme.

That seems more complex for no particular gain.  I guess it saves an
infomask bit, but I'm willing to burn one to reduce code complexity.

Robert Haas
The Enterprise PostgreSQL Company

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