> On 3 Apr 2026, at 08:42, Andrei Lepikhov <lepihov@gmail.com> wrote:
>
> Hi,
>
> I found that t/100_vacuumdb.pl has a fragile ordering check that fails if the query plan for vacuumdb's catalogue
querychanges. I sometimes see how this test fails when writing an optimisation-related extension.
>
> The test checks that vacuumdb processes "Foo".bar before "Bar".baz:
>
> qr/VACUUM\ \(SKIP_DATABASE_STATS\)\ "Foo".bar
> .*VACUUM\ \(SKIP_DATABASE_STATS\)\ "Bar".baz
> /sx,
>
> Both tables being tested, "Foo".bar and "Bar".baz, are created empty. This means pg_class.relpages is 0 for both and
thesort order is completely unstable. The output order depends entirely on which query plan will be chosen. Any change
inthe planner that affects the plan for this query, such as a new join path type or a cost model change, may flip the
orderand cause the test to fail.
>
> AFAICS, The fix is quite trivial. Change the test regex (in 100_vacuumdb.pl) to use order-independent lookaheads
insteadof a sequential match:
>
> qr/(?=.*VACUUM\ \(SKIP_DATABASE_STATS\)\ "Foo"\.bar)
> (?=.*VACUUM\ \(SKIP_DATABASE_STATS\)\ "Bar"\.baz)
> /sx,
>
> This makes the test robust regardless of the order in which the server returns results.
>
> Hence, it doesn’t change anything important. I think it deserves to be back-patched down to v.16 (like the commit
2143d96dc7bintroduced this test) so other extensions can be stable with check-world tests.
Thanks for the report, I'll have a look.
--
Daniel Gustafsson
