Our vector implementation is fully compatible with the syntax for vector fields and comparison functions in MySQL 9 -- we have carefully backported all the changes to our version of MySQL 8 to ensure full backwards and forwards compatibility. The index syntax is specific to PlanetScale because the open-source MySQL 9 preview does not yet have support for ANN indexes on vector columns, but we're committed to being as compatible as possible in the future and to minimize fragmentation. I do agree that fragmenting the ecosystem is not great!
