Re: Idea for getting rid of VACUUM FREEZE on cold pages

On Tue, Jun 8, 2010 at 2:26 AM, Stephen Frost <sfrost@snowman.net> wrote:
> Just an off-the-wall thought, but, would it be possible to have a tool
> which read WAL backwards and compared entries in the WAL against entries
> on disk?  I realize that you'd only see one version of a particular
> block and then have to skip any updates which are earlier than it, but
> it seems like you could cover a pretty large chunk of the recent changes
> to the database using this approach..

I assume you mean back out the changes incrementally until you find a
full_page_write and see if it matches? And continue comparing with
full_page_writes once per checkpoint? I don't think the WAL has enough
information to replay backwards though. For example vacuum cleanup
records just list the tids to remove. They don't have the contents to
replace there.



--
greg