code: @Override
      public boolean exists(String pid)
      throws LowlevelStorageException {
          Connection connection = null;
          PreparedStatement statement = null;
          ResultSet rs = null;
          try {
              connection = connectionPool.getReadOnlyConnection();
              statement = connection.prepareStatement(selectByIdQuery);
              statement.setString(1,pid);
              rs =
                      statement.executeQuery();
              return rs.next();
          } catch (SQLException e1) {
              throw new LowlevelStorageException(true, "sql failure (get)", e1);
          } finally {
              try {
                  if (rs != null) {
                      rs.close();
                  }
                  if (statement != null) {
                      statement.close();
                  }
                  if (connection != null) {
                      connectionPool.free(connection);
                  }
              } catch (Exception e2) { // purposely general to include uninstantiated statement, connection
                  throw new LowlevelStorageException(true,
                                                     "sql failure closing statement, connection, pool (get)",
                                                     e2);
              } finally {
                  rs = null;
                  statement = null;
                  connection = null;
              }
          }
      }
docs: Checks to see whether a pid exists in the registry.
  Makes no audits of the number of registrations or
  the paths registered.