Repo.aggregate: wrap query into a subquery if it has combinations

[smartepsh]

Environment

• Elixir version (elixir -v): 1.12.2
• Database and version (PostgreSQL 9.4, MongoDB 3.2, etc.): 13.3
• Ecto version (mix deps): master
• Database adapter and version (mix deps): postgrex 0.15.10
• Operating system: mac

Current behavior

Repo.aggregate/3 may throw an exception if query has combinations.

  supplier_query = Supplier |> select([s], s.city)
  query = Customer |> select([c], c.city) |> union(^supplier_query)

  Repo.aggregate(query, :count, :city)

  # The query will be something like 
  # Ecto.Query<from c in Customer, union: (from s in Supplier, select: s.city), select: count(c.city)>

The code above will count the Customer first , then union the count with citys in supplier_query, not count the union result.

Expected behavior

Aggregate the union result.

I wrap the query into a subquery if it has combinations .

[josevalim]

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