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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