From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
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To: | Robert Haas <robertmhaas(at)gmail(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 16:40:39 |
Message-ID: | 4CF67A87.5040101@enterprisedb.com |
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Lists: | pgsql-hackers |
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.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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