There are two implications to this change.
First, percolator now uses _uid internally, extracting the id portion
when needed. Second, sorting on _id is no longer possible, since you
can no longer index _id. However, _uid can still be used to sort, and
is better anyways as indexing _id just to make it available to
fielddata for sorting is wasteful.
see #8143closes#9842
b96bd201c1