| 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: | 4CF5FACC.6040404@enterprisedb.com |
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| 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
EnterpriseDB http://www.enterprisedb.com
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