Re: Setting pd_lower in GIN metapage

Michael Paquier <michael.paquier@gmail.com> writes:
> On Fri, Sep 8, 2017 at 5:24 AM, Tom Lane <tgl@sss.pgh.pa.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.

			regards, tom lane