Affected upgrades are Ubuntu 18.04 or lower to 18.10 or higher, so especially current LTS upgrades (18.04 to 20.04), Debian 9 or lower to Debian 10, RHEL/CentOS 7 or lower to RHEL/CentOS 8.
Not heeding the linked advices will at some point in the future lead to strange search and ordering results out of your db and will probably cause duplicate entries despite unique indices. Fixing this afterwards involves rebuilding the indexes and somehow fixing any duplicate key error you encounter.
We did not know about this beforehand as it is not noted in any docs we could see regarding distro or pg upgrades. We noticed strange things approx. 6 hours after the distro+pg upgrade with pg_upgrade and had ~10 unique index violations in our relatively small Mastodon instance, ~10 across 2 smallish HackMD instances, and ~30-40 across 2 rather lively Matrix Synapse instances. It took us around 3h to get everything sorted and can only _hope_ everything is good now. Please read the linked wiki.
Mastodon ist ein soziales Netzwerk. Es basiert auf offenen Web-Protokollen und freier, quelloffener Software. Es ist dezentral (so wie E-Mail!).