From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jeff Davis <pgsql@j-davis.com>
cc: Corey Huinker <corey.huinker@gmail.com>,
        Magnus Hagander <magnus@hagander.net>,
        Stephen Frost <sfrost@snowman.net>,
        Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>,
        Matthias van de Meent <boekewurm+postgres@gmail.com>,
        Peter Smith <smithpb2250@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
        Tomas Vondra <tomas.vondra@enterprisedb.com>
Subject: Re: Statistics Import and Export
In-reply-to: <06286ee47fa4b06b6e7f08da21958d7e35525ed4.camel@j-davis.com>
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Comments: In-reply-to Jeff Davis <pgsql@j-davis.com>
	message dated "Tue, 02 Apr 2024 14:59:12 -0700"
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Date: Tue, 02 Apr 2024 18:18:53 -0400
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Jeff Davis <pgsql@j-davis.com> writes:
> We need to get the original element type on the import side somehow,
> right? Otherwise it will be hard to tell whether '{1, 2, 3, 4}' has
> element type "int4" or "text", which affects the binary representation
> of the anyarray value in pg_statistic.

Yeah, but that problem exists no matter what.  I haven't read enough
of the patch to find where it's determining that, but I assume there's
code in there to intuit the statistics storage type depending on the
table column's data type and the statistics kind.

> Either we need to get it at export time (which seems the most reliable
> in principle, but problematic for older versions) and pass it as an
> argument to pg_set_attribute_stats(); or we need to derive it reliably
> from the table schema on the destination side, right?

We could not trust the exporting side to tell us the correct answer;
for one reason, it might be different across different releases.
So "derive it reliably on the destination" is really the only option.

I think that it's impossible to do this in the general case, since
type-specific typanalyze functions can store pretty nearly whatever
they like.  However, the pg_stats view isn't going to show nonstandard
statistics kinds anyway, so we are going to be lossy for custom
statistics kinds.

			regards, tom lane