Re: Compression of full-page-writes

On 12/08/2014 09:21 PM, Andres Freund wrote:
> I still think that just compressing the whole record if it's above a
> certain size is going to be better than compressing individual
> parts. Michael argued thta that'd be complicated because of the varying
> size of the required 'scratch space'. I don't buy that argument
> though. It's easy enough to simply compress all the data in some fixed
> chunk size. I.e. always compress 64kb in one go. If there's more
> compress that independently.

Doing it in fixed-size chunks doesn't help - you have to hold onto the
compressed data until it's written to the WAL buffers.

But you could just allocate a "large enough" scratch buffer, and give up
if it doesn't fit. If the compressed data doesn't fit in e.g. 3 * 8kb,
it didn't compress very well, so there's probably no point in
compressing it anyway. Now, an exception to that might be a record that
contains something else than page data, like a commit record with
millions of subxids, but I think we could live with not compressing
those, even though it would be beneficial to do so.

- Heikki