| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Jeff Davis <pgsql(at)j-davis(dot)com> |
| Cc: | Gokulakannan Somasundaram <gokul007(at)gmail(dot)com>, Greg Stark <stark(at)mit(dot)edu>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: An idle thought |
| Date: | 2010-03-18 21:17:15 |
| Message-ID: | 17489.1268947035@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> On Thu, 2010-03-18 at 16:50 -0400, Tom Lane wrote:
>> The VM is (a) not compressed and (b) not correctness-critical.
>> Wrong bit values don't do any serious damage.
> 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?
It doesn't. The only thing we currently rely on the VM for is deciding
whether a page needs vacuuming --- and even that we don't trust it for
when doing anti-wraparound vacuuming. The worst-case consequence of a
wrong bit is failure to free some dead tuples until the vacuum freeze
limit expires.
In order to do things like not visiting a page during scans, we'll have
to solve the reliability issues.
regards, tom lane
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