[jira] [Commented] (TINKERPOP-968) Add first class support for an optional traversal

[pieter martin (JIRA)]

    [ 
https://issues.apache.org/jira/browse/TINKERPOP-968?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15196763#comment-15196763
 ] 

pieter martin commented on TINKERPOP-968:
-----------------------------------------

Regarding multiple optional labels...

{code}
        final TinkerGraph g = TinkerGraph.open();
        Vertex a1 = g.addVertex(T.label, "A", "name", "a1");
        Vertex b1 = g.addVertex(T.label, "B", "name", "b1");
        a1.addEdge("ab", b1);

        GraphTraversal gt1 = g.traversal().V(a1).choose(__.out("ab", "bb"), 
__.out("ab", "bb"), __.identity()).path();
        while (gt1.hasNext()) {
            System.out.println(gt1.next());
        }
{code}
This will output,
{code}
[v[0], v[2]]
{code}

however what I am looking for with optional semantics is
{code}
[v[0], v[2]]
[v[0]]
{code}

This is because from {{a1}} the optional edge {{bb}} was also traversed 
returning {{a1}} as the edge {{bb}} does not exist.

So this can be done using {{union}}
{code}
        GraphTraversal gt1 = g.traversal().V(a1).union(
                __.choose(__.out("ab"), __.out("ab"), __.identity()),
                __.choose(__.out("bb"), __.out("bb"), __.identity())
        ).path();
        while (gt1.hasNext()) {
            System.out.println(gt1.next());
        }
{code}
returning 
{code}
[v[0], v[2]]
[v[0]]
{code}

All of this means that {{optional}} by itself is not powerful enough to 
complete the optional semantics. For now it will have to written as

{code}
        g.traversal().V(a1).union(
            __.optional(__.out("ab")),
            __.optional(__.out("bb"))
        )
{code}

Do you agree with my assessment?

> Add first class support for an optional traversal
> -------------------------------------------------
>
>                 Key: TINKERPOP-968
>                 URL: https://issues.apache.org/jira/browse/TINKERPOP-968
>             Project: TinkerPop
>          Issue Type: Improvement
>          Components: process
>    Affects Versions: 3.1.0-incubating
>            Reporter: pieter martin
>
> Both SparQL and Cypher use the "Optional" keyword to indicate an optional 
> traversal. SQL uses the "left join".
> Gremlin has no first class support for an optional traversal. It can be 
> achieved with the choose step but it is verbose, unintuitive and not what the 
> choose step is intended for.
> The benefits of optional traversals are many. In particular it makes it 
> trivial to load complete subgraphs/trees with one easy to read intuitive 
> gremlin statement.



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