Subject: Re: Yet another fast GiST build
To: Heikki Linnakangas <hlinnaka@iki.fi>,
 Andrey Borodin <x4mmm@yandex-team.ru>, Peter Geoghegan <pg@bowt.ie>
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Cc: Tom Lane <tgl@sss.pgh.pa.us>, Justin Pryzby <pryzby@telsasoft.com>,
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 <thomas.munro@gmail.com>, Michael Paquier <michael@paquier.xyz>,
 Alexander Korotkov <a.korotkov@postgrespro.ru>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>
From: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Organization: EnterpriseDB
Message-ID: <5b3d72ba-2522-5706-49ba-70dd90c69d3c@enterprisedb.com>
Date: Fri, 15 Jan 2021 06:24:42 +0100
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On 2021-01-12 14:49, Heikki Linnakangas wrote:
>>>> I suggest calling BuildIndexValueDescription() from your own custom
>>>> debug instrumentation code.
>>> Thanks for the hint, Peter!
>>> This function does exactly what I want to do. But I have no Relation inside gist_page_items(bytea) function... probably, I'll add gist_page_items(relname, blockno) overload to fetch keys.
>>
>> PFA patch with implementation.
> 
> I did a bit of cleanup on the function signature. The .sql script
> claimed that gist_page_items() took bytea as argument, but in reality it
> was a relation name, as text. I changed it so that it takes a page image
> as argument, instead of reading the block straight from the index.
> Mainly to make it consistent with brin_page_items(), if it wasn't for
> that precedence I might've gone either way on it.

I noticed this patch while working on another patch for pageinspect [0], 
and this one appears to introduce a problem similar to the one the other 
patch attempts to fix: The "itemlen" output parameters are declared to 
be of type smallint, but the underlying C data is of type uint16 
(OffsetNumber).  I don't know the details of gist enough to determine 
whether overflow is possible here.  If not, perhaps a check or at least 
a comment would be useful.  Otherwise, these parameters should be of 
type int in SQL.

[0]: 
https://www.postgresql.org/message-id/09e2dd82-4eb6-bba1-271a-d2b58bf6c71f@enterprisedb.com