Horton

Re: PoC/WIP: Extended statistics on expressions

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Dean Rasheed <dean.a.rasheed@gmail.com>, pgsql-hackers@postgresql.org, Michael Paquier <michael@paquier.xyz>, Peter Geoghegan <pg@bowt.ie>, Andrew Dunstan <andrew@dunslane.net>
Date: 2021-08-18T03:07:54Z
Lists: pgsql-hackers 
> 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.vondra@postgresql.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

Commits

  1. Disallow extended statistics on system columns

  2. Identify simple column references in extended statistics

  3. Don't print extra parens around expressions in extended stats

  4. Change position of field "transformed" in struct CreateStatsStmt.

  5. Add transformed flag to nodes/*funcs.c for CREATE STATISTICS

  6. Stabilize stats_ext test with other collations

  7. Extended statistics on expressions

  8. Reduce duration of stats_ext regression tests

  9. Allow composite types in catalog bootstrap

  10. Convert Typ from array to list in bootstrap

  11. Disallow CREATE STATISTICS on system catalogs