Re: [REVIEW] Re: Compression of full-page-writes

On Tue, Mar 3, 2015 at 9:24 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> On 2015-03-03 08:59:30 +0900, Michael Paquier wrote:
>> Already mentioned upthread, but I agree with Fujii-san here: adding
>> information related to the state of a block image in
>> XLogRecordBlockHeader makes little sense because we are not sure to
>> have a block image, perhaps there is only data associated to it, and
>> that we should control that exclusively in XLogRecordBlockImageHeader
>> and let the block ID alone for now.
>
> This argument doesn't make much sense to me. The flag byte could very
> well indicate 'block reference without image following' vs 'block
> reference with data + hole following' vs 'block reference with
> compressed data following'.

Information about the state of a block is decoupled with its
existence, aka in the block header, we should control if:
- record has data
- record has a block
And in the block image header, we control if the block is:
- compressed or not
- has a hole or not.
Are you willing to sacrifice bytes in the block header to control if a
block is compressed or has a hole even if the block has only data but
no image?
-- 
Michael