| From: | Greg Stark <stark(at)mit(dot)edu> |
|---|---|
| To: | Jeff Davis <pgsql(at)j-davis(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Gokulakannan Somasundaram <gokul007(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: An idle thought |
| Date: | 2010-03-18 21:28:23 |
| Message-ID: | 407d949e1003181428o44c56262v4b64e5cbbcd86b0b@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Mar 18, 2010 at 9:07 PM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> The VM cause wrong results if a bit is set that's not supposed to be --
> right? Am I missing something? How does a seq scan skip visibility
> checks and still produce right results, if it doesn't rely on the bit?
>
There's also a PD_ALL_VISIBLE flag on the page header. We wal log when
we clear that bit so we can trust if it's set then all the tuples
really are visible. I forget whether we can trust it if it's *not* set
but there's not much point -- all tuples could become visible
spontaneously even the page is sitting on disk.
--
greg
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