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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Mark Dilger <mark.dilger@enterprisedb.com>
Cc: Noah Misch <noah@leadboat.com>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Andrey M. Borodin <x4mmm@yandex-team.ru>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Amul Sul <sulamul@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: pg_amcheck contrib application
Date: Tue, 16 Mar 2021 14:45:13 -0400
Message-ID: <3387834.1615920313@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+TgmoaO8Qsn6i61f=trihm3qjcim-scNW9uq2jrUAUhydRXOQ@mail.gmail.com>
References: <A80D68F6-E38F-482D-9522-E2FB6AAFE8A1@enterprisedb.com>
	<CA+TgmoY3_U=QuQWo4DArBSp+tg2MiCHLwDN_9U2H_iTxj3UcjA@mail.gmail.com>
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	<F0A1FD70-A2F4-4528-8A03-8650CAEC0554@enterprisedb.com>
	<20210313184612.GA3195145@rfd.leadboat.com>
	<930621E5-5D10-456D-8CE4-CF629A6C4669@enterprisedb.com>
	<20210313191841.GB3195145@rfd.leadboat.com>
	<3012106.1615827854@sss.pgh.pa.us>
	<C2C142F3-F740-4197-B507-20B28B7395AC@enterprisedb.com>
	<3053552.1615834640@sss.pgh.pa.us>
	<20210316060913.GA3323238@rfd.leadboat.com>
	<7BCE9084-9381-471F-90F2-FAD99B7FE3DB@enterprisedb.com>
	<3254547.1615910855@sss.pgh.pa.us>
	<7CFAF7EF-4EA7-4CB6-B2C7-EC7F4512AE50@enterprisedb.com>
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	<CA+TgmoYFwii3cgfc-yNzEgFce=zWP08Nt56pAyffqpXjgg1UxA@mail.gmail.com>
	<3360174.1615916917@sss.pgh.pa.us>
	<CA+TgmoaO8Qsn6i61f=trihm3qjcim-scNW9uq2jrUAUhydRXOQ@mail.gmail.com>

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Mar 16, 2021 at 1:48 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> No.  What should be happening there is that some arrays in the column
>> get larger alignment than they actually need, but that shouldn't cause
>> a problem (and has not done so, AFAIK, in the decades that it's been
>> like this).  As you say, deforming the tuple is going to rely on the
>> table's tupdesc for alignment; it can't know what is in the data.

> OK, I don't understand this. attalign is 'd', which is already the
> maximum possible, and even if it weren't, individual rows can't decide
> to use a larger OR smaller alignment than expected without breaking
> stuff.

> In what context is it OK to just add extra alignment padding?

It's *not* extra, according to pg_statistic's tuple descriptor.
Both forming and deforming of pg_statistic tuples should honor
that and locate stavaluesX values on d-aligned boundaries.

It could be that a particular entry is of an array type that
only requires i-alignment.  But that doesn't break anything,
it just means we inserted more padding than an omniscient
implementation would do.

(I suppose in some parallel universe there could be a machine
where i-alignment is stricter than d-alignment, and then we'd
have trouble.)

			regards, tom lane





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