Re: PoC/WIP: Extended statistics on expressions

> Patch 0001 fixes the "double parens" issue discussed elsewhere in this
> thread, and patch 0002 tweaks CREATE STATISTICS to treat "(a)" as a simple
> column reference.

> From: Tomas Vondra <tomas(dot)vondra(at)postgresql(dot)org>
> Date: Mon, 16 Aug 2021 17:19:33 +0200
> Subject: [PATCH 2/2] fix: identify single-attribute references

> diff --git a/src/bin/pg_dump/t/002_pg_dump.pl b/src/bin/pg_dump/t/002_pg_dump.pl
> index a4ee54d516..be1f3a5175 100644
> --- a/src/bin/pg_dump/t/002_pg_dump.pl
> +++ b/src/bin/pg_dump/t/002_pg_dump.pl
> @@ -2811,7 +2811,7 @@ my %tests = (
> create_sql => 'CREATE STATISTICS dump_test.test_ext_stats_expr
> ON (2 * col1) FROM dump_test.test_fifth_table',
> regexp => qr/^
> - \QCREATE STATISTICS dump_test.test_ext_stats_expr ON ((2 * col1)) FROM dump_test.test_fifth_table;\E
> + \QCREATE STATISTICS dump_test.test_ext_stats_expr ON (2 * col1) FROM dump_test.test_fifth_table;\E

This hunk should be in 0001, no ?

> But I'm not sure 0002 is something we can do without catversion bump. What
> if someone created such "bogus" statistics? It's mostly harmless, because
> the statistics is useless anyway (AFAICS we'll just use the regular one we
> have for the column), but if they do pg_dump, that'll fail because of this
> new restriction.

I think it's okay if it pg_dump throws an error, since the fix is as easy as
dropping the stx object. (It wouldn't be okay if it silently misbehaved.)

Andres concluded similarly with the reverted autovacuum patch:
https://www.postgresql.org/message-id/20210817105022.e2t4rozkhqy2myhn@alap3.anarazel.de

+RMT in case someone wants to argue otherwise.

--
Justin