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Subject: Re: vacuumdb: add --dry-run
From: Chao Li <li.evan.chao@gmail.com>
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 <CADkLM=fo48tbJ75VPDDNgMV3f-QWLCk2TPujGP4ockxVDFRiTQ@mail.gmail.com>
Date: Tue, 18 Nov 2025 11:58:16 +0800
Cc: Nathan Bossart <nathandbossart@gmail.com>,
 pgsql-hackers@postgresql.org
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References: 
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To: Corey Huinker <corey.huinker@gmail.com>
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 <https://www.postgresql.org/message-id/3DB353E5-944D-4E18-9DA2-4151F44AA59B%40gmail.com>
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> On Nov 11, 2025, at 06:33, Corey Huinker <corey.huinker@gmail.com> =
wrote:
>=20
>=20
> My attempts to test this all got stuck in wait_on_slots().  I haven't
> looked too closely, but I suspect the issue is that the socket never
> becomes readable because we don't send a query.  If I set =
free_slot->inUse
> to false before printing the command, it no longer hangs.  We probably =
want
> to create a function in parallel_slot.c to mark slots that we don't =
intend
> to give a query as idle.
>=20
> Would that be preferable to skipping the creation of extra connections =
for parallel workers? I can see it both ways. On the one hand we want to =
give as true a reflection of "what would happen with these options", and =
on the other hand one could view the creation of extra workers as "real" =
vs a dry run.
>=20
> =20

I test the patch, but my first run was with no luck:

```
% vacuumdb --dry-run --echo -d evantest
SELECT pg_catalog.set_config('search_path', '', false);
vacuumdb: vacuuming database "evantest"
RESET search_path;
SELECT c.relname, ns.nspname FROM pg_catalog.pg_class c
 JOIN pg_catalog.pg_namespace ns ON c.relnamespace =
OPERATOR(pg_catalog.=3D) ns.oid
 CROSS JOIN LATERAL (SELECT c.relkind IN ('p', 'I')) as p (inherited)
 LEFT JOIN pg_catalog.pg_class t ON c.reltoastrelid =
OPERATOR(pg_catalog.=3D) t.oid
 WHERE c.relpersistence OPERATOR(pg_catalog.!=3D) 't'
 AND c.relkind OPERATOR(pg_catalog.=3D) ANY (array['r', 'm'])
 ORDER BY c.relpages DESC;
SELECT pg_catalog.set_config('search_path', '', false);
VACUUM (SKIP_DATABASE_STATS) pg_catalog.pg_proc; (not executed)

^CCancel request sent
^CCancel request sent
^CCancel request sent
^CCancel request sent
```

After skip the first vacuum command, the process got stuck, and ctrl-c =
couldn=E2=80=99t break it.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/