Return the Selectable Union[Table, CTE] associated with the node.

def get_node_selectable(node, context):
    """Return the Selectable Union[Table, CTE] associated with the node."""
    query_path = node.query_path
    if query_path not in context.query_path_to_selectable:
        raise AssertionError(
            u'Unable to find selectable for query path {} with context {}.'.format(
                query_path, context))
    selectable = context.query_path_to_selectable[query_path]
    return selectable