Error using incorrect cache in subquery with union

[KalvinHom]

Environment

Elixir 1.9.0
Erlang/OTP 22
Mysql 5.8
Ecto 3.1.7
MyXQL 0.2.6

Current behavior

When doing a subquery with unions, the cache is incorrectly applied if the second query in the union is cached, but the first one isn't.

defmodule Schema1 do
  use Ecto.Schema

  schema "external_payments" do
    field(:request_id, :integer)
  end
end

defmodule Schema2 do
  use Ecto.Schema

  schema "external_request_reports" do
    field(:request_id, :integer)
  end
end

defmodule Test do
  import Ecto.Query
  alias UnionTest.Repo

  def test_union() do
    test_data_1 = [1, 2, 3]
    test_data_2 = [4]

    query_1 =
      Schema1
      |> where([r], r.request_id in ^test_data_1)
      |> select([r], %Schema1{id: r.id})

    query_2 =
      Schema2
      |> where([s], s.request_id in [1, 2])
      |> select([r], %Schema2{id: r.id})
      |> distinct(true)

    subquery = Ecto.Query.subquery(union_all(query_1, ^query_2))
    Repo.all(from(a in subquery))

    query_3 =
      Schema1
      |> where([r], r.request_id in ^test_data_2)
      |> select([r], %Schema1{id: r.id})

    subquery = Ecto.Query.subquery(union_all(query_3, ^query_2))
    Repo.all(from(a in subquery))
  end
end

query_3 is trying to use query_1 's cached query, and has the wrong number of interpolated values as a result.

** (ArgumentError) parameters must be of length 3 for query %MyXQL.Query{cache: :reference, name: "ecto_679", num_params: 3, ref: #Reference<0.3417688783.2265972742.94111>, statement: "SELECT s0.`id` FROM (SELECT e0.`id` AS `id` FROM `external_payments` AS e0 WHERE (e0.`request_id` IN (?,?,?)) UNION ALL (SELECT DISTINCT e0.`id` FROM `external_payments` AS e0 WHERE (e0.`request_id` IN (1,2)))) AS s0", statement_id: 1}
    (myxql) lib/myxql/query.ex:53: DBConnection.Query.MyXQL.Query.encode/3
    (db_connection) lib/db_connection.ex:1161: DBConnection.maybe_encode/4
    (db_connection) lib/db_connection.ex:591: DBConnection.execute/4
    (ecto_sql) lib/ecto/adapters/myxql/connection.ex:28: Ecto.Adapters.MyXQL.Connection.execute/4
    (ecto_sql) lib/ecto/adapters/sql.ex:572: Ecto.Adapters.SQL.execute!/4
    (ecto_sql) lib/ecto/adapters/sql.ex:554: Ecto.Adapters.SQL.execute/5
    (ecto) lib/ecto/repo/queryable.ex:153: Ecto.Repo.Queryable.execute/4
    (ecto) lib/ecto/repo/queryable.ex:18: Ecto.Repo.Queryable.all/3

If you reverse the union order such that query_2 comes first in both unions, there is no error, I think since query_1 and query_3 are determined to not be the same cached queries first?

The key being used in Ecto.Query.Planner.query_lookup

[
  :all,
  0,
  [
    :all,
    0,
    {:union_all,
     [
       {:where,
        [
          and: {:in, [],
           [
             {{:., [], [{:&, [], [0]}, :request_id]}, [], []},
             %Ecto.Query.Tagged{
               tag: nil,
               type: {:array, {0, :request_id}},
               value: [1, 2]
             }
           ]}
        ]},
       {"external_request_reports", Schema2, 128332857, nil},
       {:select,
        {:%, [],
         [Schema2, {:%{}, [], [id: {{:., [], [{:&, [], [0]}, :id]}, [], []}]}]}},
       {:distinct, true}
     ]} |
    :nocache
  ],
  {:select, {:&, [], [0]}}
]

Expected behavior

Query should succeed and not try to use the cache.