Infer from primary_key opts if identity should be used.

Adding support for `:cache` & `:clause` settings.

lib/ecto/adapters/postgres/connection.ex

@@ -1619,26 +1619,24 @@ if Code.ensure_loaded?(Postgrex) do
     end
 
     defp column_type(:identity, opts) do
-      start_value = [Keyword.get(opts, :start_value)]
-      increment = [Keyword.get(opts, :increment)]
+      start_value = Keyword.get(opts, :start_value)
+      increment = Keyword.get(opts, :increment)
+      cache = Keyword.get(opts, :cache)
+      clause = Keyword.get(opts, :clause, :by_default) |> to_string |> String.upcase |> String.split("_") |> Enum.join(" ")
       type_name = ecto_to_db(:identity)
 
-      cleanup = fn v -> is_integer(v) and v > 0 end
 
-      sequence =
-        start_value
-        |> Enum.filter(cleanup)
-        |> Enum.map(&"START WITH #{&1}")
-        |> Kernel.++(
-          increment
-          |> Enum.filter(cleanup)
-          |> Enum.map(&"INCREMENT BY #{&1}")
-        )
+      [start_value]
+        |> Enum.filter(&(is_integer/1)) |> Enum.map(&"START WITH #{&1}")
+        |> Kernel.++( [increment] |> Enum.filter(&(is_integer/1)) |> Enum.map(&"INCREMENT BY #{&1}") )
+        |> Kernel.++( [cache] |> Enum.filter(&(is_integer(&1) && Kernel.>(&1, 0))) |> Enum.map(&"CACHE #{&1}"))
+        |> case do
+          [] -> []
+          [h | t] -> [["(", h], [t, ") "]]
+        end
+        |> (&([type_name, " GENERATED #{clause} AS IDENTITY", &1 |> Enum.join(" ")])).()
+
 
-      case sequence do
-        [] -> [type_name, " GENERATED BY DEFAULT AS IDENTITY"]
-        _ -> [type_name, " GENERATED BY DEFAULT AS IDENTITY(", Enum.join(sequence, " "), ") "]
-      end
     end
 
     defp column_type(type, opts) do

lib/ecto/migration.ex

@@ -420,7 +420,7 @@ defmodule Ecto.Migration do
             name: String.t(),
             prefix: atom | nil,
             comment: String.t() | nil,
-            primary_key: boolean | keyword(),
+            primary_key: boolean | keyword() |  [start_value: integer(), increment: integer(), cache: pos_integer(), clause: :by_default | :always],
             engine: atom,
             options: String.t()
           }
@@ -1595,8 +1595,9 @@ defmodule Ecto.Migration do
 
   defp pk_opts_to_tuple(opts) do
     opts = Keyword.put(opts, :primary_key, true)
+    inferred_type = [:start_value, :increment, :cache, :clause] |> Enum.any?(&Keyword.has_key?(opts, &1)) && :bigint || :bigserial
     {name, opts} = Keyword.pop(opts, :name, :id)
-    {type, opts} = Keyword.pop(opts, :type, :bigserial)
+    {type, opts} = Keyword.pop(opts, :type, inferred_type)
     {name, type, opts}
   end
 end