Re: Quick-and-dirty compression for WAL backup blocks

On Mon, 6 Jun 2005, Tom Lane wrote:

> Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
>> Vacuum doesn't zero out the free space between lower and upper,
>
> It does now ;-)

Oh :). Does it affect vacuum performance?

>> How about adding a flag to XLogRecData to indicate if the space between
>> pd_lower and pd_upper is meaningful or not? The XLogInsert caller probably
>> knows that. That way you could completely skip over the free space if
>> it's not meaningful, saving even more cycles.
>
> Hmm ... that might not be a bad idea. As far as I can think offhand,
> all the XLogInsert callers know very well what type of page they are
> working with, so they would always be able to set such a flag correctly.
>
> Would this be institutionalizing a particular approach to data
> compression in the XLogInsert API, though?

The "skip the free space" optimization is still useful and worthwhile
even if we have a more sophisticated compression method for the
rest of the page.

- Heikki