Subject: Re: analyze-in-stages post upgrade questions

From: Adrian Klaver <[email-masked]>
Date: Fri, 27 Jun 2025 08:31:08 -0700

On 6/27/25 06:35, Zechman, Derek S wrote:
> We recently performed an upgrade from pg14 (14.18) to pg16 (16.9) and 
> performed the analyze-in-stages post upgrade.  It has been noticed that 
> some plans changed to use hash joins instead of nested loops.  Further 
> investigation found it was because the parent table of partitioned 
> tables did not have stats.  After running an ANALYZE on the parent 
> tables we got similar plan an execution times as before.
> 
> I have two questions
> 
> 1 - Why does analyze-in-stages not analyze the parent tables?
> 
> 2 – What happens if we do not run analyze-in-stages post upgrade and 
> just run an analyze?

It is spelled out in the docs:

https://www.postgresql.org/docs/current/pgupgrade.html

Emphasis added

"Using vacuumdb --all --analyze-only can efficiently generate such 
statistics, and the use of --jobs can speed it up. Option 
--analyze-in-stages can be used to generate **minimal statistics** 
quickly. If vacuum_cost_delay is set to a non-zero value, this can be 
overridden to speed up statistics generation using PGOPTIONS, e.g., 
PGOPTIONS='-c vacuum_cost_delay=0' vacuumdb ...."

and from here:

https://www.postgresql.org/docs/current/app-vacuumdb.html

"--analyze-in-stages

     Only calculate statistics for use by the optimizer (no vacuum), 
like --analyze-only. Run three stages of analyze; the first stage uses 
the lowest possible statistics target (see default_statistics_target) to 
produce usable statistics faster, and subsequent stages build the full 
statistics.

     This option is only useful to analyze a database that currently has 
no statistics or has wholly incorrect ones, such as if it is newly 
populated from a restored dump or by pg_upgrade. Be aware that running 
with this option in a database with existing statistics may cause the 
query optimizer choices to become transiently worse due to the low 
statistics targets of the early stages.
"

> Thanks,
> Sean

-- 
Adrian Klaver
[email-masked]

---

From: Laurenz Albe <[email-masked]>
Date: Fri, 27 Jun 2025 23:39:17 +0200

> statistics targets of the early stages.

Well, that wouldn't explain why it doesn't work on partitioned tables.
I am under the impression that it should.

Derek, can cou share the pg_stats entries for the partitioned table?

Yours,
Laurenz Albe

---

From: "Zechman, Derek S" <[email-masked]>
Date: Sat, 28 Jun 2025 01:23:59 +0000

> https://urldefense.com/v3/__https://www.postgresql.org/docs/current/pgupgrade.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvnFFR6Ws$<https://urldefense.com/v3/__https:/www.postgresql.org/docs/current/pgupgrade.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvnFFR6Ws$>

> https://urldefense.com/v3/__https://www.postgresql.org/docs/current/app-vacuumdb.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvvC7gfd0$<https://urldefense.com/v3/__https:/www.postgresql.org/docs/current/app-vacuumdb.html__;!!Lf_9VycLqA!jU5QgA1re0Txg_h2dD1N3XvK_l8hYdyMvpcxrLL5GDyQ5qN4aGQcxmDE8qmaV_p5telTzYmOL6S3fR8eRc0s_8UvvC7gfd0$>

Yours,

Laurenz Albe

There are no entries in pg_stats for the parent table until after I manually run an analyze on it – Example below

=> select relname, reltuples, relkind from pg_class where relname ~ '^chapter_[0-9]+$' or relname='chapter' order by 1;

   relname   | reltuples | relkind

-------------+-----------+---------

 chapter     |        -1 | p

 chapter_1   |         4 | r

 chapter_10  |         4 | r

 chapter_100 |        30 | r

 chapter_101 |        15 | r

 chapter_102 |        15 | r

…

=> select count(*) from pg_stats where tablename='chapter';

 count

-------

     0

(1 row)

=> analyze chapter;

ANALYZE

=> select relname, reltuples, relkind from pg_class where relkind ='p' and relname='chapter';

 relname | reltuples | relkind

---------+-----------+---------

 chapter |      7589 | p

(1 row)

=> select count(*) from pg_stats where tablename='chapter';

 count

-------

    49

(1 row)

toy_epc_stg_1_db=>