计算AQL中遍历的不同节点

Question

I am able to get all distinct nodes from a query, but not the count:

FOR v in 2..2 OUTBOUND "starting_node" GRAPH "some_graph"
return DISTINCT v._key

I want to get only the count of the result. I tried to use LENGTH(DISTINCT v._key) as suggested in the docs , but it's not a proper syntax of the AQL:

syntax error, unexpected DISTINCT modifier near 'DISTINCT v._key)'

The naive solution is to get all keys and count it on the client side, but I am curious how to do it on the server side?

Answer 1

What RETURN DISTINCT does is to remove duplicate values , but only after the traversal.

You can set traversal options to eliminate paths during the traversal, which can be more efficient especially if you have a highly interconnected graph and a high traversal depth:

RETURN LENGTH(
  FOR v IN 2..2 OUTBOUND "starting_node" GRAPH "some_graph"
  OPTIONS { uniqueVertices: "global", bfs: true }
    RETURN v._key
)

The traversal option uniqueVertices can be set to "global" so that you don't get the same vertex returned twice from this traversal. The option for breadth-first search bfs needs to be enabled to use uniqueVertices: "global" . The reason why depth-first search does not support this uniqueness option is that the result would not be deterministic, hence this combination was disabled.

Answer 2

Inspired by this blogpost http://jsteemann.github.io/blog/2014/12/12/aql-improvements-for-24/ I prepared the solution using LET :

LET result = (FOR v in 2..2 OUTBOUND "starting_node" GRAPH "some_graph"
                  return DISTINCT v._key)
RETURN LENGTH(result)

It might be not optimal solution, but it works as I expected.