Re: Setting pd_lower in GIN metapage

On Sat, Sep 9, 2017 at 9:00 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Michael Paquier <michael(dot)paquier(at)gmail(dot)com> writes:
>> On Fri, Sep 8, 2017 at 5:24 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> In short, this patch needs a significant rewrite, and more analysis than
>>> you've done so far on whether there's actually any benefit to be gained.
>>> It might not be worth messing with.
>
>> I did some measurements of the compressibility of the GIN meta page,
>> looking at its FPWs with and without wal_compression and you are
>> right: there is no direct compressibility effect when setting pd_lower
>> on the meta page. However, it seems to me that there is an argument
>> still pleading on favor of this patch for wal_consistency_checking.
>
> I think that would be true if we did both my point 1 and 2, so that
> the wal replay functions could trust pd_lower to be sane in all cases.
> But really, if you have to touch all the places that write these
> metapages, you might as well mark them REGBUF_STANDARD while at it.
>
>> The same comment ought to be mentioned for btree.
>
> Yeah, I was wondering if we ought not clean up btree/hash while at it.
> At the very least, their existing comments saying that it's inessential
> to set pd_lower could use some more detail about why or why not.
>

+1. I think we can even use REGBUF_STANDARD in the hash for metapage
where currently it is not used. I can give a try to write a patch for
hash/btree part if you want.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com