From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Michael Paquier <michael(dot)paquier(at)gmail(dot)com> |
Cc: | Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Setting pd_lower in GIN metapage |
Date: | 2017-09-09 15:30:05 |
Message-ID: | 24903.1504971005@sss.pgh.pa.us |
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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.
regards, tom lane
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