Moving to Hibernate ORM.
Even though you’re using Hibernate Search, this error clearly is related to Hibernate ORM, and Hibernate Search does not care (at all) what dialect you use to communicate with your database.
Regarding your error… I would bet on a configuration problem in Spring Boot.
Make sure you don’t create the EntityManagerFactory/SessionFactory yourself, but leave that to Spring Boot. You mentioned here that you declared an EntityManagerFactory bean, and I’m almost certain this wasn’t a good idea.
See my answer to solve the problem you had before declaring that bean.