database,performance,neo4j,cypher , Check previously visited nodes on cypher


Check previously visited nodes on cypher

Question:

Tag: database,performance,neo4j,cypher

I have a big dataset and I am doing the following query to find paths between two nodes:

MATCH (a:Industry{ id:140 }),(b:Industry{ id:386 })
WITH a,b
MATCH p=(a)-[:SELLS*..3]->(b)
return p

I heard that cypher does DFS, which is ok for me. But my graph has many loops, so there are many cases when for example a node at (tree)level 3 has a connection with a node at level 1, which results in much more checks than needed.

How can I check if a node has been visited already before visiting it? Is it even possible?


Answer:

Honestly I don't know whether cypher does a DFS or not - and you shouldn't know or care either.

One of the big points of advantage with cypher is that it's a declarative graph query language. That is, you tell it what data you want and it goes and gets it. You don't have to know or care how, and hopefully with time as a query optimizer gets better, it's going to end up doing it faster than you could have anyway. Contrast this with imperative graph query, which is where you specify how you want the data to be fetched.

Cypher generally doesn't retrace paths it's already been over because that wouldn't be efficient, but it might visit certain nodes multiple times depending on the query (i.e. if there are multiple paths through to the same node).

Cypher doesn't provide any features for controlling how the graph traversal happens - hopefully you'll agree with me that this is a feature, not a bug.

If you want fine grained control over how the graph is traversed, what has been seen, or what to do when things are encountered, then I think you want the java Traversal API. Among many other options, you can specify BFS/DFS, and you can specify traversal options like "only visit each node once" (no matter what the relationships specify) or the flip, "only visit each relationship once".

Now, to the case of your particular query, if it's returning many more paths than you want, perhaps you need to narrow your query to get rid of the ones you don't want. You might want to look at longest path queries for example to get rid of the "shortcut" paths through the hierarchy.


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