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