Introduction:
    The line "$this->rowsIndexByLabel[$label] = count($this->rows)-1;" in function
"public function addRow( Piwik_DataTable_Row $row )" doesn't work correctly.
For example:

$rows ( 1=>"AA", 2=>"BB", 3=>"CC", 4=>"DD", 5=>"EE", 6=>"FF");
note:1=>"AA": "1" is the index id of label "AA"

delete $rows[3], we get:

$rows ( 1=>"AA", 2=>"BB", 4=>"DD", 5=>"EE", 6=>"FF");

addRow($newrow),the label of $newrow is "GG"

The result should be:
$rows ( 1=>"AA", 2=>"BB", 4=>"DD", 5=>"EE", 6=>"FF", 7=>"GG");

while according to function addRow(), the index id of $newrow's label("GG") is 

$this->rowsIndexByLabel[$label] = count($this->rows)-1 =5-1=4,

but the right index id of label "GG" is 7.

We find the function rebuildIndex() useless after we change the function addRow().

The code change contains the following three points:

1¡¢£¨1£©original code(4 functions):
    public function addRow( Piwik_DataTable_Row $row )
	{
		$this->rows[] = $row;
		
		if(!$this->indexNotUpToDate
			&& $this->rebuildIndexContinuously)
		{
			$label = $row->getColumn('label');
			if($label !== false)
			{
				$this->rowsIndexByLabel[$label] = count($this->rows)-1;				
			}
			$this->indexNotUpToDate = false;
		}
	}
	
	static public function isEqual(Piwik_DataTable $table1, Piwik_DataTable $table2)
	{
		$rows1 = $table1->getRows();
		$rows2 = $table2->getRows();
		
		$table1->rebuildIndex();
		$table2->rebuildIndex();
		
		if($table1->getRowsCount() != $table2->getRowsCount())
		{
			return false;
		}
		
		foreach($rows1 as $row1)
		{
			$row2 = $table2->getRowFromLabel($row1->getColumn('label'));
			if($row2 === false
				|| !Piwik_DataTable_Row::isEqual($row1,$row2))
			{
				return false;
			}
		}		
		return true;
	}
	
	public function getRowFromLabel( $label )
	{
		$this->rebuildIndexContinuously = true;
		if($this->indexNotUpToDate)
		{
			$this->rebuildIndex();
		}
		
		if($label === self::LABEL_SUMMARY_ROW
			&& !is_null($this->summaryRow))
		{
			return $this->summaryRow;
		}
		
		$label = (string)$label;
		if(!isset($this->rowsIndexByLabel[$label]))
		{
			return false;
		}
		return $this->rows[$this->rowsIndexByLabel[$label]];
	}
	
		public function sort( $functionCallback, $columnSortedBy )
	{
		$this->indexNotUpToDate = true;
		$this->tableSortedBy = $columnSortedBy;
		usort( $this->rows, $functionCallback );
		
		if($this->enableRecursiveSort === true)
		{
			foreach($this->getRows() as $row)
			{
				if(($idSubtable = $row->getIdSubDataTable()) !== null)
				{
					$table = Piwik_DataTable_Manager::getInstance()->getTable($idSubtable);
					$table->enableRecursiveSort();
					$table->sort($functionCallback, $columnSortedBy);
				}
			}
		}
	}
	

    £¨2£©changed code(4 functions):
    public function addRow( Piwik_DataTable_Row $row )
	{
		$this->rows[] = $row;
		
		$label = $row->getColumn('label');
		if($label !== false)
		{
			$this->rowsIndexByLabel[$label] = $this->nextRowId;
			$this->nextRowId++;
		}
	}
	
	static public function isEqual(Piwik_DataTable $table1, Piwik_DataTable $table2)
	{
		$rows1 = $table1->getRows();
		$rows2 = $table2->getRows();
		
		
		if($table1->getRowsCount() != $table2->getRowsCount())
		{
			return false;
		}
		
		foreach($rows1 as $row1)
		{
			$row2 = $table2->getRowFromLabel($row1->getColumn('label'));
			if($row2 === false
				|| !Piwik_DataTable_Row::isEqual($row1,$row2))
			{
				return false;
			}
		}		
		return true;
	}

	public function getRowFromLabel( $label )
	{
		if($label === self::LABEL_SUMMARY_ROW
			&& !is_null($this->summaryRow))
		{
			return $this->summaryRow;
		}
		
		$label = (string)$label;
		if(!isset($this->rowsIndexByLabel[$label]))
		{
			return false;
		}
		return $this->rows[$this->rowsIndexByLabel[$label]];
	}
	
		public function sort( $functionCallback, $columnSortedBy )
	{
		$this->tableSortedBy = $columnSortedBy;
		usort( $this->rows, $functionCallback );
		
		if($this->enableRecursiveSort === true)
		{
			foreach($this->getRows() as $row)
			{
				if(($idSubtable = $row->getIdSubDataTable()) !== null)
				{
					$table = Piwik_DataTable_Manager::getInstance()->getTable($idSubtable);
					$table->enableRecursiveSort();
					$table->sort($functionCallback, $columnSortedBy);
				}
			}
		}
	}	
	
2¡¢added codes:	
	protected $nextRowId = 0;	
3¡¢deleted code:
	protected $indexNotUpToDate = true;
    protected $rebuildIndexContinuously = false;		 
	private function rebuildIndex()
	{
		foreach($this->rows as $id => $row)
		{
			$label = $row->getColumn('label');
			if($label !== false)
			{
				$this->rowsIndexByLabel[$label] = $id;
			}
		}
		$this->indexNotUpToDate = false;
	}