Re: VACUUM (DISABLE_PAGE_SKIPPING on)

On Fri, Nov 20, 2020 at 9:08 AM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> There are two costs associated with this processing.  One is dirtying
> the page (which means it needs to be written down when evicted), and the
> other is to write WAL records for each change.  The cost for the latter
> is going to be the same in both cases (with this change and without)
> because the same WAL will have to be written -- the only difference is
> *when* do you pay it.  The cost of the former is quite different; with
> Simon's patch we dirty the page once, and without the patch we may dirty
> it several times before it becomes "stable" and no more writes are done
> to it.
>
> (If you have tables whose pages change all the time, there would be no
> difference with or without the patch.)
>
> Dirtying the page less times means less full-page images to WAL, too,
> which can be significant.

Yeah, I think dirtying the page fewer times is a big win. However, a
page may have tuples that are not yet all-visible, and we can't freeze
those just because we are freezing others.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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