Re: vacuumdb: add --dry-run

Nathan Bossart <nathandbossart@gmail.com>

From: Nathan Bossart <nathandbossart@gmail.com>

To: Corey Huinker <corey.huinker@gmail.com>

Cc: pgsql-hackers@postgresql.org

Date: 2025-11-10T21:19:23Z

Lists: pgsql-hackers

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Add ParallelSlotSetIdle().
- 750816971b35 19 (unreleased) landed
vacuumdb: Add --dry-run.
- d107176d27c7 19 (unreleased) landed
vacuumdb: Move some variables to the vacuumingOptions struct.
- cf1450e57799 19 (unreleased) landed
Log a note at program start when running in dry-run mode
- c05dee191125 19 (unreleased) cited

On Mon, Nov 10, 2025 at 02:44:41PM -0500, Corey Huinker wrote:
> This is a small patch to add a new option to vacuumdb to answer the
> question "what commands will actually be run by this combination of
> command-line switches against this database?" without actually running the
> commands.

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.

-- 
nathan