change_column_null

# File activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb, line 418
        def change_column_null(table_name, column_name, null, default = nil)
          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_value(default, column)} WHERE #{quote_column_name(column_name)} IS NULL") if column
          end
          execute("ALTER TABLE #{quote_table_name(table_name)} ALTER #{quote_column_name(column_name)} #{null ? 'DROP' : 'SET'} NOT NULL")
        end