Re: Test to dump and restore objects left behind by regression

On Thu, Feb 22, 2024 at 8:35 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Peter Eisentraut <peter@eisentraut.org> writes:
> > The problem is, we don't really have any end-to-end coverage of
>
> > dump
> > restore
> > dump again
> > compare the two dumps
>
> > with a database with lots of interesting objects in it.
>
> I'm very much against adding another full run of the core regression
> tests to support this.

This will be taken care of by Peter's latest idea of augmenting
existing 002_pg_upgrade.pl.

> But beyond the problem of not bloating the
> check-world test runtime, there is the question of what this would
> actually buy us.  I doubt that it is worth very much, because
> it would not detect bugs-of-omission in pg_dump.  As I remarked in
> the other thread, if pg_dump is blind to the existence of some
> feature or field, testing that the dumps compare equal will fail
> to reveal that it didn't restore that property.
>
> I'm not sure what we could do about that.  One could imagine writing
> some test infrastructure that dumps out the contents of the system
> catalogs directly, and comparing that instead of pg_dump output.
> But that'd be a lot of infrastructure to write and maintain ...
> and it's not real clear why it wouldn't *also* suffer from
> I-forgot-to-add-this hazards.

If a developer forgets to add logic to dump objects that their patch
adds, it's hard to detect it, through testing alone, in every possible
case. We need reviewers to take care of that. I don't think that's the
objective of this test case or of pg_upgrade test either.

>
> On balance, I think there are good reasons that we've not added
> such a test, and I don't believe those tradeoffs have changed.
>

I am not aware of those reasons. Are they documented somewhere? Any
pointers to the previous discussion on this topic? Googling "pg_dump
regression pgsql-hackers" returns threads about performance
regressions.

On the flip side, the test I wrote reproduces the COMPRESSION/STORAGE
bug you reported along with a few other bugs in that area which I will
report soon on that thread. I think, that shows that we need such a
test.

-- 
Best Wishes,
Ashutosh Bapat