Gremlin: What's an efficient way of finding an edge between two vertices?

Question

So obviously, a straight forward way to find an edge between two vertices is to:

graph.traversal().V(outVertex).bothE(edgeLabel).filter(__.otherV().is(inVert         


        

Answer 1

I would expect the first query to be faster. However, few things:

1. None of the queries is optimal, since both of them enable path calculations. If you need to find a connection in both directions, then use 2 queries (I will give an example below)
2. When you use clock(), be sure to iterate() your traversals, otherwise you'll only measure how long it takes to do nothing.

These are the queries I would use to find an edge in both directions:

g.V(a).outE(edgeLabel).filter(inV().is(b))
g.V(b).outE(edgeLabel).filter(inV().is(a))

If you expect to get at most one edge:

edge = g.V(a).outE(edgeLabel).filter(inV().is(b)).tryNext().orElseGet {
       g.V(b).outE(edgeLabel).filter(inV().is(a)).tryNext()
}

This way you get rid of path calculations. How those queries perform will pretty much depend on the underlying graph database. Titan's query optimizer recognizes that query pattern and should return a result in almost no time.

Now if you want to measure the runtime, do this:

clock(100) {
  g.V(a).outE(edgeLabel).filter(inV().is(b)).iterate()
  g.V(b).outE(edgeLabel).filter(inV().is(a)).iterate()
}

Answer 2

In case one does not know the vertex Id's, another solution might be

g.V().has('propertykey','value1').outE('thatlabel').as('e').inV().has('propertykey','value2').select('e')

This is also only unidirectional so one needs to reformulate the query for the opposite direction.