The musicbrainz_unaccent function calls the unaccent extension underneath with a hardcoded dictionary parameter, such that it can be marked as IMMUTABLE. But as the comment above it warns, "changing the unaccent dictionary on the filesystem can still break the IMMUTABLE assumption."

In fact, if the unaccent.rules file is updated alongside a PostgreSQL upgrade, then indexes using musicbrainz_unaccent can become corrupted as the behavior of the function may change (while the indexes were built using the old behavior). This has manifested in issues like MBS-13576.

We should provide a script (similar to the one MBS-13336 offered for collations) which allows rebuilding indexes using the musicbrainz_unaccent function, intended to be run after PostgreSQL upgrades.