On 01.12.2010 15:39, Bruce Momjian wrote:
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?
If we update the LSN when we set the flag, then we have to write the
full-page-image. That's very expensive. If we don't update the LSN, then
the LSN says nothing about when the flag was set.
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.
Perhaps we could live with it, but it would be annoying to have to
checkpoint after a data load, before index-only scans kick in.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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