Consecutive repeat-times-emit-dedup steps inside an and/or step

[Eric Steele]

Hi all,

I recently joined this group. I'm loving Gremlin so far, but I'm having trouble explaining some results I'm seeing. Any help you can provide would be much appreciated!

Implementation

I am using DataStax Graph 5.0.2

The Setup

My graph includes 7 vertices. Each vertex has a property named "value" that is an uppercase letter: A, B, C, D, E, F, and G. This graph is arranged as follows:

A             F
 ╲           ╱
  C -> D -> E
 ╱           ╲
B             G

From left to right, all edges shown above are outgoing edges (A and B point to C; C points to D; D points to E; E points to F and G).

Given the following query:

g.V().hasLabel('label').or(
  __.in('edge').out('edge').has('value', 'G'),                        // traversal #1
  __.in('edge').in('edge').out('edge').has('value', 'G'),             // traversal #2

  __.in('edge').out('edge').out('edge').has('value', 'G'),            // traversal #3

  __.in('edge').in('edge').out('edge').out('edge').has('value', 'G')  // traversal #4

)

I get back vertices E, F, and G, which makes sense to me. Traversals #1 and #4 both yield F and G; traversal #3 yields E; traversal #2 yields nothing

I figured that I could also represent the query above as follows:

g.V().hasLabel('label').or(
  __.repeat(__.in('edge')).times(2).emit().dedup().out().has('value', 'G'),       // traversal #5
  __.repeat(__.in('edge')).times(2).emit().dedup().out().out().has('value', 'G')  // traversal #6
)

As I expected, this query also returns vertices E, F, and G. 

I then tried to see if the following query could achieve the same result:

g.V().hasLabel('label').or(
  __.repeat(__.in('edge')).times(2).emit().dedup().repeat(out('edge')).times(2).emit().dedup().has('value', 'G') // traversal #7
)

This, however, returns vertices A, C, E, F and G, which does not make sense to me. Can anyone explain why this query returns vertices A and C?

My expectation was that the final ".has('value', 'G')" clause would filter out A and C because both are at least 3 hops away from G, and the "repeat(out('edge'))" is only "times(2)".

Furthermore, when I execute the *inverse* of this query (i.e. reverse edge directions and target vertex A):

g.V().hasLabel('label').or(
  __.repeat(out('edge')).times(2).emit().dedup().repeat(__.in('edge')).times(2).emit().dedup().has('value', 'A') // traversal #8
)

I am getting back vertices A, B, and C, which I would expected. There seems to be a discrepancy here, given that this query is the same as query #3, just inverted.

I'd be eternally grateful if someone could shed some light on this situation.

Best Regards,
Eric Steele

[Eric Steele]

For anyone that wants to test this out for themselves, here you can run the following to get the graph setup:

Vertex A = graph.addVertex(label, 'label', 'value', 'A');

Vertex B = graph.addVertex(label, 'label', 'value', 'B');

Vertex C = graph.addVertex(label, 'label', 'value', 'C');

Vertex D = graph.addVertex(label, 'label', 'value', 'D');

Vertex E = graph.addVertex(label, 'label', 'value', 'E');

Vertex F = graph.addVertex(label, 'label', 'value', 'F');

Vertex G = graph.addVertex(label, 'label', 'value', 'G');

A.addEdge('edge', C);

B.addEdge('edge', C);

C.addEdge('edge', D);

D.addEdge('edge', E);

E.addEdge('edge', F);

E.addEdge('edge', G);

[Eric Steele]

If my understanding of Gremlin is correct, traversal #7 should be more-or-less equivalent to the following:

g.V().hasLabel('label').has('value', 'G').repeat(__.in('edge')).times(2).emit().dedup().repeat(out('edge')).times(2).emit().dedup()

Which returns E, F, and G as expected. Again, this makes me wonder why #7 returns A, E, F, and G.

[Eric Steele]

One correction to make: Traversals #1 yields only G, #4 yields both F and G; traversal #3 yields E; traversal #2 yields nothing

g.V().hasLabel('label').or(
  __.in('edge').out('edge').has('value', 'G'),                        // traversal #1
  __.in('edge').in('edge').out('edge').has('value', 'G'),             // traversal #2

  __.in('edge').out('edge').out('edge').has('value', 'G'),            // traversal #3

  __.in('edge').in('edge').out('edge').out('edge').has('value', 'G')  // traversal #4

)

[HadoopMarc]

Hi Eric,

Did not try to understand, but just put it in my v3.2.3 gremlin console:

gremlin> g.V().hasLabel('label').or(__.repeat(__.in('edge')).times(2).emit().dedup().repeat(out('edge')).times(2).emit().dedup().has('value', 'G')).valueMap()
==>[value:[E]]
==>[value:[F]]
==>[value:[G]]
gremlin> g.V().hasLabel('label').valueMap()
==>[value:[A]]
==>[value:[B]]
==>[value:[C]]
==>[value:[D]]
==>[value:[E]]
==>[value:[F]]
==>[value:[G]]

So, I get E,F, G for traversal 7, as you wanted.

Hope this points you in the right direction!

Marc

Op donderdag 2 maart 2017 20:57:42 UTC+1 schreef Eric Steele:

[Daniel Kuppitz]

Same here, nothing unexpected. Just wanted to add a traversal, that helps to debug the result:

gremlin> g.withPath().V().hasLabel('label').or(
           repeat(__.in('edge')).times(2).emit().dedup().repeat(out('edge')).times(2).emit().dedup().has('value', 'G').
           sideEffect {println it.path().objects()*.value("value")}
         ).values()
[E, D, E, G]
==>E
[F, E, G]
==>F
[G, E, G]
==>G

Cheers,

Daniel

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[Eric Steele]

Thank you both for giving it a try. At the very least, I think this shows that my reasoning was correct. 

Perhaps there's a bug in my installation of DSE Graph. I'll try updating and see if the issue persists.

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[Eric Steele]

Hey, sorry to bother you again, but when I run the Gremlin you posted:

gremlin> Gremlin.version

==>3.2.4-20170124-6a0001ac

gremlin> g.withPath().V().hasLabel('label').or(

gremlin>           repeat(__.in('edge')).times(2).emit().dedup().repeat(out('edge')).times(2).emit().dedup().has('value', 'G').

gremlin>           sideEffect {println it.path().objects()*.value("value")}

gremlin>          ).values()

==>E

==>G

==>A

==>F

I'm not seeing the full path printed as you do. It just shows the vertices. Is there something more I need to do?

On Thursday, March 2, 2017 at 5:44:28 PM UTC-8, Daniel Kuppitz wrote:

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[Daniel Kuppitz]

If you're using Gremlin Server, then you won't see the printed paths in your local console. You should be able to find it in the server logs though.

Cheers,

Daniel

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