support subqueries in order_bys

test/ecto/adapters/postgres_test.exs

@@ -618,6 +618,17 @@ defmodule Ecto.Adapters.PostgresTest do
     assert all(query) ==
              ~s{SELECT s0."x" FROM "schema" AS s0 ORDER BY s0."x" ASC NULLS FIRST, s0."y" DESC NULLS FIRST}
 
+    order_by =
+     [asc: Ecto.Query.dynamic([], exists(from other_schema in "schema", where: other_schema.x == parent_as(:r).x, select: [other_schema.x]))]
+
+    query =
+      (from row in Schema, as: :r)
+      |> order_by(^order_by)
+      |> select([r], r.x)
+      |> plan()
+
+    assert all(query) == ~s{SELECT s0."x" FROM "schema" AS s0 ORDER BY exists((SELECT ss0."x" AS "result" FROM "schema" AS ss0 WHERE (ss0."x" = s0."x")))}
+
     query =
       Schema
       |> order_by([r], asc_nulls_last: r.x, desc_nulls_last: r.y)