Neo4j: error in adding relationships among existing nodes

Question

When writing a query to add relationships to existing nodes, it keeps me warning with this message:

"This query builds a cartesian product between disconnected patterns. If a part of a query contains multiple disconnected patterns, this will build a cartesian product between all those parts. This may produce a large amount of data and slow down query processing. While occasionally intended, it may often be possible to reformulate the query that avoids the use of this cross product, perhaps by adding a relationship between the different parts or by using OPTIONAL MATCH (identifier is: (e))"

If I run the query, it creates no relationships. The query is:

match   
    (a{name:"Angela"}),
    (b{name:"Carlo"}),
    (c{name:"Andrea"}),
    (d{name:"Patrizia"}),
    (e{name:"Paolo"}),
    (f{name:"Roberta"}),
    (g{name:"Marco"}),
    (h{name:"Susanna"}),
    (i{name:"Laura"}),
    (l{name:"Giuseppe"})


create  
    (a)-[:mother]->(b),
    (a)-[:grandmother]->(c), (e)-[:grandfather]->(c), (i)-[:grandfather]->(c), (l)-[:grandmother]->(c),
    (b)-[:father]->(c),
    (e)-[:father]->(b),
    (l)-[:father]->(d),
    (i)-[:mother]->(d),
    (d)-[:mother]->(c),
    (c)-[:boyfriend]->(f),
    (g)-[:brother]->(f),
    (g)-[:brother]->(h),
    (f)-[:sister]->(g), (f)-[:sister]->(h)

Can anyone help me?

PS: if I run the same query, but with just one or two relationships (and less nodes in the match clause), it creates the relationships correctly.

Answer 1

What is wrong here?

• First of all, as I mentionned in my comments, you don't have any Labels, it's a really bad practice because Labels are useful to match properties in a certains dataset (if you match "name" property, you don't want to match it on a node who doesn't have a name, Labels are here for that.

• The second problem is that your query doesn't know how many nodes it will get before it does. It means that if you have 500 000 nodes having name : "Angela" and 500 000 nodes having name : "Carlo", you will create one relation from each Angela node, going on each Carlo, that's quite a big query (500 000 * 500 000 relations to create if my maths aren't bad). Cypher is giving you a warning for that.

• Cypher will still tell you this warning because you aren't using Unique properties to match your nodes, even with Labels, you will still have the warning.

Solution?

• Use unique properties to create and match your nodes, so you avoid cartesian product.

• Always use labels, Neo4j without labels is like using one giant table in SQL to store all of your data.

If you want to know how your query will run, use PROFILE before your query, here is the profile plan for your query:

Answer 2

Does every single one of those name strings exist? If not then you're not going to get any results because it's all one big match. You could try changing it to a MERGE.

But Supamiu is right, you really should have a label (say Person) and an index on :Person(name).