Multiple left joins with aggregation and unique vindexes can't route plan

[dweitzman]

I'm going to mail a PR that adds this test case documenting the limitation. Not sure what it would take to fix it. Query planning would succeed if one of the left joins were removed:

# Multi-table unique vindex constraint on left table of left join, two levels of join, simple aggregation
# This should work, but doesn't. If you remove the join with ue2 this works, but for some reason when
# there are two levels of left join instead of just one the query planner fails to realize that all these
# rows must belong to the same shard.
"select user.id, count(*) from user left join user_extra ue1 on user.id = ue1.user_id left join user_extra ue2 on ue1.user_id = ue2.user_id group by user.id"
"unsupported: cross-shard query with aggregates"

[dweitzman]

There's a workaround, incidentally, which is adding "and user.id = ue2.user_id" to the second "on" clause. That should be implied from user.id = ue1.user_id && ue1.user_id = ue2._user_id, though, since both of those use unique vindexes.