Re: Some other things about contrib/bloom and generic_xlog.c

On Mon, Apr 11, 2016 at 11:29 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Michael Paquier <michael.paquier@gmail.com> writes:
>> Actually, as I look at this code for the first time, I find that
>> GenericXLogFinish() is a very confusing interface. It makes no
>> distinction between a page and the meta data associated to this page,
>> this is controlled by a flag isNew and buffer data is saved as some
>> delta. Actually, fullmage is not exactly true, this may refer to a
>> page that has a hole. It seems to me that we should not have one but
>> two routines: GenericXLogRegisterBuffer and
>> GenericXLogRegisterBufData, similar to what the normal XLOG routines
>> offer.
>
> Hmm.  Maybe the documentation leaves something to be desired, but
> I thought that the interface was reasonable if you grant that the
> goal is to be easy to use rather than to have maximum performance.
> The calling code only has to concern itself with making the actual
> changes to the target pages, not with constructing WAL descriptions
> of those changes.  Also, the fact that the changes don't have to be
> made within a critical section is a bigger help than it might sound.
>
> So I don't really have any problems with the API as such.  I tried
> to improve the docs a day or two ago, but maybe that needs further
> work.

Well, I am not saying that the given interface does nothing, far from
that. Though I think that we could take advantage of the rmgr
RM_GENERIC_ID introduced by this interface to be able to generate
custom WAL records and pass them through a stream.

As given out now, we are able to do the following:
- Log a full page
- Log a delta of a full page, which is actually data associated to a page.
- At recovery, replay those full pages with a delta.

What I thought we should be able to do with that should not be only
limited to indexes, aka to:
- Be able to register and log full pages
- Be able to log data associated to a block
- Be able to have record data
- Use at recovery custom routines to apply those WAL records
The first 3 ones are done via the set of routines generating WAL
records for the rmgr RM_GENERIC_ID. The 4th one needs a hook in
generic_redo. Looking at the thread where this patch has been debated
I am not seeing a discussion regarding that.
-- 
Michael