I also read the Spring docs, as lapkritinis suggested - and luckily this brought me on the right path! But I don´t think, that the Spring docs explain this good right now. At least for me, they aren´t consistent IMHO.
The original problem/question is on what to do, if you upgrade an existing Spring Boot 1.5.x application to 2.0.x, which is using PostgreSQL/Hibernate. The main reason, you get your described error, is that Spring Boot 2.0.x uses HikariCP instead of Tomcat JDBC pooling DataSource as a default - and Hikari´s DataSource doesn´t know the spring.datasource.url property, instead it want´s to have spring.datasource.jdbc-url (lapkritinis also pointed that out).
So far so good. BUT the docs also suggest - and that´s the problem here - that Spring Boot uses spring.datasource.url to determine, if the - often locally used - embedded Database like H2 has to back off and instead use a production Database:
You should at least specify the URL by setting the spring.datasource.url property. Otherwise, Spring Boot tries to auto-configure an embedded database.
You may see the dilemma. If you want to have your embedded DataBase like you´re used to, you have to switch back to Tomcat JDBC. This is also much more minimally invasive to existing applications, as you don´t have to change source code! To get your existing application working after the Spring Boot 1.5.x --> 2.0.x upgrade with PostgreSQL, just add tomcat-jdbc as a dependency to your pom.xml:
<dependency>
<groupId>org.apache.tomcat</groupId>
<artifactId>tomcat-jdbc</artifactId>
</dependency>
And then configure Spring Boot to use it accordingly inside application.properties:
spring.datasource.type=org.apache.tomcat.jdbc.pool.DataSource
Hope to help some folks with this, was quite a time consuming problem. I also hope my beloved Spring folks update the docs - and the way new Hikari pool is configured - to get a more consistent Spring Boot user experience :)