Re: pgsql: Trial fix for old cross-version upgrades.

On 2025-02-22 Sa 9:20 PM, Tom Lane wrote:
> I wrote:
>> Furthermore, it can't be a coincidence that the four tables we are
>> seeing relallvisible diffs for are exactly the four tables in the
>> regression database that have hash indexes.
> Oh! If the source version is <= 9.6, old_9_6_invalidate_hash_indexes()
> generates a script "reindex_hash.sql" to reindex all hash indexes.
> And TestUpgradeXversion faithfully executes that, *before* making
> the new-database comparison dump.
>
> I added some instrumentation and confirmed that these tables'
> relallvisible values match between the pre-dump state on the old
> database and the immediately-post-upgrade state on the new database.
> It's definitely reindex_hash.sql that's changing them. This doesn't
> in itself explain why 9.3-9.5 don't show the problem, but I noticed
> something interesting: it's the pre-dump state that is out of line
> in 9.2 and 9.6. All the other cases show relallvisible that's a
> couple pages less than relpages, but in those two branches we
> start from a state that claims these rels are fully all-visible,
> and then seemingly REINDEX discovers that that's not so.
> What I'm guessing is that the variance is due to changes in
> vacuum/analyze's heuristics for updating pg_class.relallvisible
> after a partial scan of the table.
>
> Anyway, we know where the culprit is, and I'm not sure that
> explaining the differences in behavior of long-dead branches
> is an exciting use of time.
>
> Question is, what to do about this? We probably want to continue to
> test that reindex_hash.sql does something sane, so just deleting that
> step won't do. I experimented with moving it from immediately before
> the pg_dumpall of the new installation to immediately after, but that
> didn't work at all: the indexes are marked invalid and so pg_dump just
> doesn't dump them, so we still end up with a diff in the dump output.
>
> I'm not really seeing a better answer than hacking the comparison
> rules to ignore the relallvisible difference for these tables.
> That's darn ugly, and I suspect the implementation will be messy,
> but do we have another way?
>
>

Having slept on it I can't see anything better. It's only for very old
branches, and nothing is really going wrong here, so ignoring the
difference seems quite reasonable.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com