Is there any efficient way to get all relationship types currently defined?
I know this works:
match ()-[r]-() RETURN DISTINCT TYPE(r)
But I guess this will consume significant time if the number of relationships is huge and there is no inherent indexing under the hood.
To learn more about what mysteries a graph holds see
How to get a high level inventory of objects in your graph
At https://neo4j.com/download/ you can down load Neo4j desktop which will install a local copy of Neo4j server. Here is the online guide.
In there is a list of sample queries to help you learn about a database.