| From: | Corey Huinker <corey(dot)huinker(at)gmail(dot)com> |
|---|---|
| To: | Nathan Bossart <nathandbossart(at)gmail(dot)com> |
| Cc: | Jeff Davis <pgsql(at)j-davis(dot)com>, Robert Treat <rob(at)xzilla(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(at)paquier(dot)xyz>, jian he <jian(dot)universality(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Stephen Frost <sfrost(at)snowman(dot)net>, Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>, Peter Smith <smithpb2250(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, alvherre(at)alvh(dot)no-ip(dot)org |
| Subject: | Re: Statistics Import and Export |
| Date: | 2025-04-03 02:34:58 |
| Message-ID: | CADkLM=d4U6hjoLGx2tRxLzSsEDYz7oj8PmmTYPKmnfPk5RLzKQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
>
> > Also, why do we need the clause "WHERE s.tablename = ANY($2)"? Isn't
> > that already implied by "JOIN unnest($1, $2) ... s.tablename =
> > u.tablename"?
>
> Good question. Corey, do you recall why this was needed?
>
In my patch, that SQL statement came with the comment:
+ /*
+ * The results must be in the order of relations supplied in the
+ * parameters to ensure that they are in sync with a walk of the TOC.
+ *
+ * The redundant (and incomplete) filter clause on s.tablename = ANY(...)
+ * is a way to lead the query into using the index
+ * pg_class_relname_nsp_index which in turn allows the planner to avoid an
+ * expensive full scan of pg_stats.
+ *
+ * We may need to adjust this query for versions that are not so easily
+ * led.
+ */
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