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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@heroku.com>
Cc: 大塚憲司 <otsuka.kenji@nttcom.co.jp>
Cc: pgsql-docs@postgresql.org, Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] The number of bytes is stored in index_size of pgstatindex() ?
Date: Thu, 18 Feb 2016 19:53:53 -0500
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Peter Geoghegan <pg@heroku.com> writes:
> On Thu, Feb 18, 2016 at 4:15 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Because they've been removed from the right-link/left-link chains.

> That isn't the same thing as being inaccessible by scans, clearly
> (just what you call the "leaf scan sequence").

Only a physical-order scan, ie vacuum, would visit a dead page
(ignoring transient corner cases like a page getting deleted while an
indexscan is in flight to it).  So I think treating it as part of the
fragmentation measure is completely wrong: the point of that measure,
AFAICS, is to model how close an index-order traversal is to linear.
Half-dead pages are also normally very transient --- the only way they
persist is if there's a crash partway through a page deletion.  So I think
it's appropriate to assume that future indexscans won't visit those,
either.

> there are usage patterns where half-dead pages might accumulate.

Other than a usage pattern of "randomly SIGKILL backends every few
seconds", I don't see how that would happen.

			regards, tom lane


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