Method reorder

/**
 * Method to compact the ordering values of rows in a group of rows defined by an SQL WHERE clause.
 *
 * @param   string  $where  WHERE clause to use for limiting the selection of rows to compact the ordering values.
 *
 * @return  mixed  Boolean  True on success.
 *
 * @since   1.7.0
 * @throws  \UnexpectedValueException
 */
public function reorder($where = '')
{
    // Check if there is an ordering field set
    if (!$this->hasField('ordering')) {
        throw new \UnexpectedValueException(sprintf('%s does not support ordering.', \get_class($this)));
    }
    $quotedOrderingField = $this->_db->quoteName($this->getColumnAlias('ordering'));
    $subquery = $this->_db->getQuery(true)->from($this->_tbl)->selectRowNumber($quotedOrderingField, 'new_ordering');
    $query = $this->_db->getQuery(true)->update($this->_tbl)->set($quotedOrderingField . ' = sq.new_ordering');
    $innerOn = array();
    // Get the primary keys for the selection.
    foreach ($this->_tbl_keys as $i => $k) {
        $subquery->select($this->_db->quoteName($k, 'pk__' . $i));
        $innerOn[] = $this->_db->quoteName($k) . ' = sq.' . $this->_db->quoteName('pk__' . $i);
    }
    // Setup the extra where and ordering clause data.
    if ($where) {
        $subquery->where($where);
        $query->where($where);
    }
    $subquery->where($quotedOrderingField . ' >= 0');
    $query->where($quotedOrderingField . ' >= 0');
    $query->innerJoin('(' . (string) $subquery . ') AS sq ');
    foreach ($innerOn as $key) {
        $query->where($key);
    }
    // Pre-processing by observers
    $event = AbstractEvent::create('onTableBeforeReorder', ['subject' => $this, 'query' => $query, 'where' => $where]);
    $this->getDispatcher()->dispatch('onTableBeforeReorder', $event);
    $this->_db->setQuery($query);
    $this->_db->execute();
    // Post-processing by observers
    $event = AbstractEvent::create('onTableAfterReorder', ['subject' => $this, 'where' => $where]);
    $this->getDispatcher()->dispatch('onTableAfterReorder', $event);
    return true;
}