On Mon, Jun 6, 2011 at 12:19 AM, Itagaki Takahiro
<itagaki.takahiro@gmail.com> wrote:
> On Sun, Jun 5, 2011 at 12:03, Robert Haas <robertmhaas@gmail.com> wrote:
>> If other buffer pins do exist, then we can't
>> defragment the page, but that doesn't mean no useful work can be done:
>> we can still mark used line pointers dead, or dead line pointers
>> unused. We cannot defragment, but that can be done either by the next
>> VACUUM or by a HOT cleanup.
>
> This is just an idea -- Is it possible to have copy-on-write techniques?
> VACUUM allocates a duplicated page for the pinned page, and copy valid
> tuples into the new page. Following buffer readers after the VACUUM will
> see the cloned page instead of the old pinned one.
Heikki suggested the same thing, and it's not a bad idea, but I think
it would be more work to implement than what I proposed. The caller
would need to be aware that, if it tries to re-acquire a content lock
on the same page, the offset of the tuple within the page might
change. I'm not sure how much work would be required to cope with
that possibility.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
