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Re: crash-safe visibility map, take three

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: crash-safe visibility map, take three
Date: 2010-12-01 13:39:07
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Heikki Linnakangas wrote:
> On 01.12.2010 03:35, Bruce Momjian wrote:
> > Heikki Linnakangas wrote:
> >> Let's recap what happens when a VM bit is set: You set the
> >> PD_ALL_VISIBLE flag on the heap page (assuming it's not set already, it
> >> usually isn't), and then set the bit in the VM while keeping the heap
> >> page locked.
> >
> > What if we set PD_ALL_VISIBLE on the heap page, wait for a checkpoint to
> > happen so the heap page is guaranteed to be on disk, then on next read,
> > if PD_ALL_VISIBLE is set and the VM all-visible bit is not set, set the
> > VM bit.
> Hmm, you'd somehow have to know if a checkpoint has happened since the 
> flag was set. And it might be a long wait, which makes it less 

Well, doesn't the page LSN tell you that already?

> attractive for index-only scans.

My assumption is that this page will remain read-only for a while, so I
don't see the big downside in a delay of max 5-10 minutes.  For sites
where pages go frequently in and out of read-only status, I don't think
index-only scans are every going to be a big win.  Long-running queries
alone are going to delay how quickly we can set PD_ALL_VISIBLE, so I
don't see an additional 5-10 minutes as a big problem.

  Bruce Momjian  <bruce(at)momjian(dot)us>

  + It's impossible for everything to be true. +

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