Re: Setting pd_lower in GIN metapage

On Sun, Sep 10, 2017 at 3:15 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> 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.

Coordinating efforts here would be nice. If you, Amit K, are taking
care of a patch for btree and hash, would you, Amit L, write the part
for GIN, BRIN and SpGist? This needs a careful lookup as many code
paths need a lookup so it may take time. Please note that I don't mind
double-checking this part if you don't have enough room to do so.
--
Michael