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

On 6/8/2010 8:27 AM, Greg Stark wrote:
> On Tue, Jun 8, 2010 at 2:26 AM, Stephen Frost <sfrost(at)snowman(dot)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.

You can't back out changes. WAL does not contain before images.

Jan

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