Re: Setting pd_lower in GIN metapage

On Wed, Sep 20, 2017 at 12:47 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:
>> On Tue, Sep 19, 2017 at 9:27 AM, Michael Paquier
>> <michael(dot)paquier(at)gmail(dot)com> wrote:
>>> I am not saying that no index AMs take advantage FPW compressibility
>>> for their meta pages. There are cases like this one, as well as one
>>> code path in BRIN where this is useful, and it is useful as well when
>>> logging FPWs of the init forks for unlogged relations.
>
>> Hmm, why is it useful for logging FPWs of the init forks for unlogged
>> relations? We don't use REGBUF_STANDARD in those cases.
>
> But if we started to do so, that would be a concrete benefit of this
> patch ...

In the proposed set of patches, all the empty() routines part of index
AMs which use log_newpage_buffer() (brin, gin, spgst) are doing the
right thing by updating log_newpage_buffer(). btree also should have
its call to log_newpage updated in btbuildempty(), and your patch is
missing that. Also, _hash_init() would need some extra work to
generate FPWs, but I don't think that it is necessary per its handling
of a per-record meta data either. So REGBUF_STANDARD could be just
removed from there, and there is actually no need to patch
src/backend/access/hash at all.
--
Michael