Support limit for predicates

[ghost]

limit() is available for RangeVertexQuery, PipeVertexQuery and PipeEdgeQuery. Is it possible for limit() to be supported for predicates such as PipePropertyValueEdgeQuery, PipePropertyValueVertexQuery, PipePropertyPresenseEdgeQuery and PipePropertyPresenseVertexQuery too? Here is an example of where it may be useful:

// get all vertices of type "my_type" then get the first one with the property "Name" of value "John".
let q = RangeVertexQuery::new()
        .t(my_type)
        .with_property_equal_to(Identifier::new("Name").unwrap(), "John".into())
        .limit(1) // <- get only the first vertex with the property above
        .outbound() // traverse onwards...
        ...

While I'm not familiar with rocksdb or the TAO approach, while using Tinkerpop Gremlin performance could be gained by limiting traversals where they were no longer needed. In an example such as the one above, if I'm sure that only a single vertex matches the predicate, or my query only needs a single result, then the query need not look for more vertices, and it can continue to the next step, outbound().

Another example:

// Does the property "Salary" exist on any vertex within the database?
let q = PropertyPresenceVertexQuery::new(Identifier::new("Salary").unwrap()).limit(1);

[ysimonson]

Yup this makes sense

[FibreFoX]

Maybe this should be split into two separate things.

1. .limit() for limits more than just 1; always returns a list (coming from Java, sorry if "list" is not the correct terminology for Rust)
2. .exists() as a better way to check for anything being there (always returns boolean)

What do you think about this? Probably would avoid the situation having .limit(1) returning a different thing than .limit(2).

[ysimonson]

Yeah, an exists() also makes sense.