    # File lib/active_record/connection_adapters/postgresql_adapter.rb, line 887
          def change_column_null(table_name, column_name, null, default = nil)
            unless null || default.nil?
              execute("UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote(default)} WHERE #{quote_column_name(column_name)} IS NULL")
            end
            execute("ALTER TABLE #{quote_table_name(table_name)} ALTER #{quote_column_name(column_name)} #{null ? 'DROP' : 'SET'} NOT NULL")
          end