Version 2.28 (release notes) of the GNU C library introduces many changes to the collations it provides. Collations determine how strings are compared and by default, PostgreSQL uses the operating system’s collations which on Linux means glibC. When your operating system updates to this version of glibc and you aren't using the “C” or “POSIX” collation, you may encounter some differently ordered indexes. This unexpected change in the order of indexes will lead to incorrectly ordered query results and possible data corruption. Currently, the following distributions are affected:
For most major upgrades using a utility such as pg_upgrade or a replication tool such as pglogical will be the best solution. However if these options are not available, pg_dumpall can be used to safely perform a major upgrade of your PostgreSQL database.