Re: [HACKERS] Setting pd_lower in GIN metapage

On Fri, Jun 23, 2017 at 11:17 AM, Amit Langote
<Langote_Amit_f8@lab.ntt.co.jp> wrote:
> That was it, thanks for the pointer.

GinInitMetabuffer() sets up pd_lower and pd_upper anyway using
PageInit so the check of PageIsVerified is guaranteed to work in any
case. Upgraded pages will still have their pd_lower set to the
previous values, and new pages will have the optimization. So this
patch is actually harmless for past pages, while newer ones are seen
as more compressible.

> Attached updated patch, which I confirmed, passes wal_consistency_check = gin.

I have spent some time looking at this patch, playing with pg_upgrade
to check the state of the page upgraded. And this looks good to me.
One thing that I noticed is that this optimization could as well
happen for spgist meta pages. What do others think?
-- 
Michael