Indices on Element label

[phreed]

This is mostly a performance question.
Why don't labels have indices?
A reasonably large database takes a long time to evaluate a query of the form...

g.V().hasLabel('foo')...toList()

I have modified my database to have, for every vertex label, a VertexLabel property.
This makes it so that the query above can be evaluated equivalently as...

g.V().has('VertexLabel','foo')...toList()

However, this second form has an index on VertexLabel and performs quickly.

[li-boxuan]

Feel free to join the discussions on #283

Your approach looks good to me, but just be careful that if your index is a composite index, it could lead to the hot key problem when you have millions of nodes of the same label.