Finding cypher paths that don't visit the same node twice

问题

I'm looking for the paths between two nodes in a graph, but my graph has a loop in it and so I'm getting paths back that are undesirable. I'm hoping that someone here can help me think out a sensible remedy.

Here's my graph:

A
|
2
|
C-3-D
|   |
|   4
5   |
|   E
|   |
|   6
|   |
F-7-G

The letters are nodes, and the numbers are edges (relationships).

CREATE (a {i: "A"})
CREATE (c {i: "C"})
CREATE (d {i: "D"})
CREATE (e {i: "E"})
CREATE (f {i: "F"})
CREATE (g {i: "G"})
CREATE a-[:r {i:2}]->c-[:r {i:3}]->d-[:r {i:4}]->e-[:r {i:6}]->g
CREATE c-[:r {i:5}]->f-[:r {i:7}]->g;

I'm looking for paths between A and C, and I'd only expect there to be one, but there are three!

neo4j-sh (?)$ MATCH p=({i: "a"})-[:r*]-({i: "c"}) return EXTRACT(n IN NODES(p) | n.i);
+-------------------------------+
| EXTRACT(n IN NODES(p) | n.i)  |
+-------------------------------+
| ["A","C"]                     |
| ["A","C","D","E","G","F","C"] |
| ["A","C","F","G","E","D","C"] |
+-------------------------------+

neo4j-sh (?)$ MATCH p=({i: "a"})-[:r*]-({i: "c"}) return EXTRACT(n IN RELATIONSHIPS(p) | n.i);
+--------------------------------------+
| EXTRACT(n IN RELATIONSHIPS(p) | n.i) |
+--------------------------------------+
| [2]                                  |
| [2,3,4,6,7,5]                        |
| [2,5,7,6,4,3]                        |
+--------------------------------------+

That makes sense from a graph perspective because the path isn't visiting the same edge twice, but from a node perspective it's a pain because C clearly is visited twice.

One thought was to try shortest paths, using allShortestPaths, however that only appears to filter the result by returning just the shortest length paths, which isn't the same as avoiding passing through the same node twice. For example the route A->G has two paths:

"A" -> "G" : [[2, 5, 7], [2, 3, 4, 6]]

but when I use allShortestPaths I only get the three hop path [2,5,7].

Is there a sensible way of applying a restriction so that I only get paths where each node is only visited once?

回答1:

I think you should use shortestPath or allShortestPaths, like this:

MATCH p=shortestPath((:Label1 {i: "a"})-[:r*]-(:Label2 {i: "c"})) 
RETURN EXTRACT(n IN NODES(p) | n.i);

make sure to create an index/constraint for :Label(i)

you can try something like that (filter out all paths where a node appears twice)

MATCH p=({ i: "A" })-[:r*]-({ i: "C" })
WHERE NONE (n IN nodes(p) 
            WHERE size(filter(x IN nodes(p) 
                              WHERE n = x))> 1)
RETURN EXTRACT(n IN RELATIONSHIPS(p)| n.i);

the index hint was an optimization for a real world dataset

来源：https://stackoverflow.com/questions/28261198/finding-cypher-paths-that-dont-visit-the-same-node-twice

标签

neo4j

cypher