Thanks for your input!

The solution I came up with is to use a CTE to hold the dynamic values from the unnest:

union_query = 
  "cte_name"
  |> with_cte("cte_name", as: fragment("select my_column from unnest(ARRAY[5, 10, 15]) my_column"))
  |> select([c], %{ my_column: fragment("my_column") }

This query is able to be unioned

It’s been some time since this was posted but with the latest Ecto v3.9 at least there’s a way to do it without a CTE (although not too pretty either). Can’t say about previous Ecto versions.

data = [1,2,3]
q1 = 
  from(data in fragment(“select mycolumn from unnest(?::int[]) as x(mycolumn)”, ^data),
    select: data.mycolumn
   )
q2 = from(table in MyTable, select: table.mycolumn)
q3 = union_all(q1, ^q2)
values = Repo.all(q3)