make the identity type a singleton to save on object creation

activerecord/lib/active_record/relation/calculations.rb

@@ -154,9 +154,7 @@ def pluck(*column_names)
         result = klass.connection.select_all(relation.arel, nil, bind_values)
         columns = result.columns.map do |key|
           klass.column_types.fetch(key) {
-            result.column_types.fetch(key) {
-              Class.new { def type_cast(v); v; end }.new
-            }
+            result.column_types.fetch(key) { result.identity_type }
           }
         end
 

activerecord/lib/active_record/result.rb

@@ -8,6 +8,8 @@ module ActiveRecord
   class Result
     include Enumerable
 
+    IDENTITY_TYPE = Class.new { def type_cast(v); v; end }.new # :nodoc:
+
     attr_reader :columns, :rows, :column_types
 
     def initialize(columns, rows, column_types = {})
@@ -17,6 +19,10 @@ def initialize(columns, rows, column_types = {})
       @column_types = column_types
     end
 
+    def identity_type # :nodoc:
+      IDENTITY_TYPE
+    end
+
     def each
       if block_given?
         hash_rows.each { |row| yield row }