ArangoDB: Aggregating counts via graph traversal

Question

In my ArangoDB graph, I have a subject, message threads associated with that subject, and messages inside those message threads. I would like to traverse the graph in such a way that I return the data associated with the message thread as well as the count of messages inside the message thread.

The data is structured fairly simply: I have the subject node, an edge extending to the thread node with the date and category associated, and an edge from the thread node to the message node.

I would like to return the data stored in the thread node and the count of messages attached to the thread.

I'm not sure how to do this with the for v, e, p in 1..2 outbound syntax. Should I just do for v, e, p in outbound with a nested graph inside it? Is that still performant?

Answer 1

Sorry for the delay, we are working hard on 3.1 release ;)

I think you are already at the correct solution: It is not easily possible to express what you would like to achieve in a 1..2 OUTBOUND statement. It is way easier to formulate in two 1..1 OUTBOUND statements.

From your explanation i think the following query is what you would use:

FOR thread IN 1 OUTBOUND @start @@threadEdges
  LET nr = COUNT(FOR message IN 1 OUTBOUND thread @@messageEdges RETURN 1)
  RETURN {
    date: thread.date,
    category: thread.category,
    messages: nr
  }

For some explanation: i first select the associated thread. Next i do a subquery to simply could the messages for one thread. Finally i return the information i need.

In terms of performance: In terms of data access (which is Most likely the "bottleneck" operation) there is no difference in FOR x IN 1..2 OUTBOUND [...] and FOR x IN 1 OUTBOUND [...] FOR y IN 1 OUTBOUND x [...] both have to look at exactly the same documents. The query optimization might be a bit slower in the later case, but the difference is way below 1ms.