Neo4j Relationship Schema Indexes

Question

Using Neo4j 2.1.4 and SDN 3.2.0.RELEASE

I have a graph that connects nodes with relationships that have a UUID associated with them. External systems use the UUID as a means to identify the source and target of the relationship. Within Spring Data Neo4j (SDN) we have a @RelationshipEntity(type=”LINKED_TO”) class with a @StartNode, @EndNode and a String uuid field. The uuid field is @Indexed and the resulting schema definition in Neo4j shows up as

neo4j-sh (?)$ SCHEMA
==> Indexes
...
==>   ON :Link(uuid) ONLINE
...

However, running a cypher query against the data, e.g.

MATCH ()-[r:LINKED_TO]->() WHERE uuid=’XXXXXX’ RETURN r;

does a full scan of the database and takes a long time

If I try to use the index by running

MATCH ()-[r:LINKED_TO]->() USING INDEX r:Link(uuid) WHERE uuid=’XXXXXX’ RETURN r;

I get

SyntaxException: Type mismatch: expected Node but was Relationship.

As I understand it, Relationships are supposed to be first class citizens in Neo4j, but I can’t see how to utilize the index on the relationship to prevent the graph equivalent of a table scan on the database to locate the relationship.

I know there are posts like How to use relationship index in Cypher which ask similar things, but this Link is the relationship between the two nodes. If I converted the Link to a Node, we would be creating a Node to represent a Relationship which seems wrong when we are working in a graph database - I'd end up with ()-[:xxx]->(:Link)-[:xxx]->() to represent one relationship. It would make the model messy purely due to the fact that the Link couldn't be represented as a relationship.

The Link has got a unique, shared key attached to it that I want to use. The Schema output suggests that the index is there for that field - I just can't use it.

Does anyone have any suggestions?

Many thanks,

Dave

I don't get it. Is your uuid a node property or a relationship property? According to your schema, you have an index on :Link(uuid), where Link is a label which can only exist on nodes. But in your second query, you're putting the label next to a relationship (which also explains your SyntaxException). — Jan Van den bosch
The uuid is a relationship property. I think there is some confusion as adding an @Indexed(unique=true) String uuid; onto the @RelationshipEntity(type="LINKED_TO") public class Link results in SDN executing create constraint on (n:Link) assert n.uuid is unique which, as you say, is actually creating a constraint on a Node with label Link - hence why it doesn't work. I think that's a bug in SDN - it shouldn't allow @Indexed on a @RelationshipEntity. But there's still the whole question about how to index the property on the Link relationship. — Dave Hallam

Stefan Armbruster Stefan Armbruster · Accepted Answer · 2014-09-27T12:06:54

Schema indexes are only available for nodes. The only way to index relationships is using legacy indexes or autoindexes. Legacy indexes need to be used explicitly in the START clause:

START r=relationship:my_index_name(uuid=<myuuid>)
RETURN r

I'm not sure how this can be used in conjunction with SDN.

side note: requiring relationship index is almost always a indication of doing something wrong in your graph data model. Everything being a thing or having an identity in your domain should be a node. So if a relationship requires a uuid, maybe the relationship refers to a thing and therefore should be converted into a node having a inbound relationship to the previous start node and a outbound relationship to the previous end node.

Neo4j Relationship Schema Indexes

1 Answers