Detecting unused database columns using Ecto schemas - https://geekmonkey.org/detecting-unused-database-columns-using-ecto-schemas/

schema_checker.ex

defmodule SchemaChecker do
  def find_field_discrepancies do
    schema_modules()
    |> Enum.map(fn mod -> 
      {mod.__schema__(:source), check_module(mod)}
    end)
  end

  defp schema_modules do
    {:ok, modules} = :application.get_key(:core, :modules)
    
    modules 
    |> Enum.filter(&({:__schema__, 1} in &1.__info__(:functions))) 
    |> Enum.filter(&(:__meta__ in Map.keys(&1.__schema__(:loaded))))
  end
  
  defp check_module(module) do
    fields = 
      module.__schema__(:fields)
      |> Enum.map(&module.__schema__(:field_source, &1))
      |> Enum.map(&Atom.to_string/1)
      
    table_name = module.__schema__(:source)

    {:ok, %{columns: columns}} = DB.Repo.query("SELECT * FROM #{table_name} LIMIT 0")

    columns -- fields
  end
end

SchemaChecker.find_field_discrepancies