change_column_null

# File activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb, line 414
        def change_column_null(table_name, column_name, null, default = nil) # :nodoc:
          clear_cache!
          unless null || default.nil?
            column = column_for(table_name, column_name)
            execute "UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote_default_expression(default, column)} WHERE #{quote_column_name(column_name)} IS NULL" if column
          end
          execute "ALTER TABLE #{quote_table_name(table_name)} #{change_column_null_for_alter(table_name, column_name, null, default)}"
        end