Causes all steps prior to the barrier step to be executed before moving onto additional steps.
barrier() step is a bulk optimizing step that can create a single traverser for a repeated step.
Repeat this traversal with the
barrier() step and then without, to see the effect the barrier step has on the operation.
The basic query starts at a particular book, then traverses to the includedIn edges that point to that book to
g.V().has('book','bookId',1004). in('includedIn'). in('includedIn'). groupCount('x'). by('name'). barrier(). project('a','b'). by('name'). by(select('x'))