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Re: crash-safe visibility map, take three

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
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 07:35:40
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
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 
attractive for index-only scans.

   Heikki Linnakangas

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