def psql_command(sql_command)
      Preconditions.assert_class(sql_command, String)
      command = "psql --no-align --tuples-only --no-psqlrc --command \"%s\" %s" % [sql_command, @url]
      Library.system_or_error(command)
    end
def psql_command(sql_command)
      Preconditions.assert_class(sql_command, String)
      command = "psql --no-align --tuples-only --no-psqlrc --command \"%s\" %s" % [sql_command, @url]
      Library.system_or_error(command)
    end
executes a simple sql command.