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Re: crash-safe visibility map, take three

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2010-11-30 17:10:49
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> We're not going to double the cost of VACUUM to get index-only scans.
> And that's exactly what will happen if you do full-page writes of
> every heap page to set a single bit.

It's ridiculous to claim that that "doubles the cost of VACUUM".  In the
worst case, it will add 25% to the cost of setting an all-visible bit on
a page where there is no other work to do.  (You already are writing out
the heap page and the VM page, plus a WAL image of the heap page, so a
WAL image of the VM page adds 25%.  But only if you did not set any
other bits on the same VM page, which is probably not a real common
case.)  Given that VACUUM has a lot of other cleanup besides visibility
bit setting, I'm not convinced that this would even be noticeable.

I think the burden is on people who are proposing complicated mechanisms
to show that there's actually a strong need for them.

			regards, tom lane

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Subject: Re: crash-safe visibility map, take three
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