just grep the AST for OuterJoin nodes rather than converting the tree to SQL

activerecord/lib/active_record/relation/calculations.rb

@@ -166,7 +166,7 @@ def perform_calculation(operation, column_name, options = {})
       if operation == "count"
         column_name ||= (select_for_count || :all)
 
-        if arel.join_sql =~ /LEFT OUTER/i
+        unless arel.ast.grep(Arel::Nodes::OuterJoin).empty?
           distinct = true
           column_name = @klass.primary_key if column_name == :all
         end