Re: vacuumdb: add --dry-run - Mailing list pgsql-hackers

On Fri, 5 Dec 2025 at 02:52, Nathan Bossart <nathandbossart@gmail.com> wrote:
>
> On Thu, Nov 20, 2025 at 04:16:13PM -0600, Nathan Bossart wrote:
> > On Thu, Nov 20, 2025 at 05:09:54PM -0500, Corey Huinker wrote:
> >> I have no objections to, but I am curious about the factors that went into
> >> making dry_run an independent boolean rather than part of vacopts.
> >
> > My thinking was that it's closer to "echo" and "quiet" than anything in
> > vacuumingOptions.  Most of that stuff seems geared towards controlling the
> > precise behavior of the commands rather than the behavior of the
> > application.  TBH I think it'd be fine either way.  We could probably even
> > move "echo" and "quiet" into vacuumingOptions if we really wanted to.
>
> Yeah, I'm finding myself liking the idea of moving all of these things into
> vacuumingOptions so that we don't have to cart around so many bool
> arguments.  Here's a new patch set that does it this way.
>
> The remaining question in my mind is where we should let the user know that
> we're in dry-run mode.  The three options I see are 1) at the beginning of
> vacuumdb execution, 2) in the !quiet block for each database, and 3) in
> each command (via a comment).  In v5, I've added a message to all three,
> but I'm eager to hear what folks think.
>
> --
> nathan


Hi!

1)
>  +     <varlistentry>
> +      <term><option>--dry-run</option></term>
> +      <listitem>
> +       <para>
> +        Print, but do not execute, the vacuum and analyze commands that would
> +        have been sent to the server (performs a dry run).
> +       </para>
> +      </listitem>
> +     </varlistentry>


I compared this smgl section to analogous sections of server utilities
(pg_dump, pg_resetwal, pg_rewind), none of them mentions "dry run" in
description of what "--dry run" is. So, I think my feeling is that
this `(performs a dry run).` is unneeded is correct.

2)

>  - printf(_("%s: vacuuming database \"%s\"\n"),
> -   progname, PQdb(conn));
> + printf(_("%s: vacuuming database \"%s\"%s\n"),
> +   progname, PQdb(conn),
> +   vacopts->dry_run ? " (dry-run)" : "");


I am also not sure we need this change. Look:

```
reshke@yezzey-cbdb-bench:~/pg$ ./bin/bin/vacuumdb --dry-run
vacuumdb: Executing in dry-run mode.
vacuumdb: vacuuming database "reshke" (dry-run)
```

We have two lines which say the same. Well, maybe there is value in
this change, if we are vacuuming multiple databases, but given that
--dry-run produces a lot of
`VACUUM ...  -- not executed` output, I think It will be obvious that
this vacuumdb run does not modify the system. WDYT?

Overall, 0001 and 0003 are ok, I don't have an opinion on 0002.

-- 
Best regards,
Kirill Reshke