Bug 145991 [Regression]Invalid data set should not be allowed to drag or "Insert in layout"
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/DataSetHandle.java
* @see org.eclipse.birt.report.model.api.elements.structures.ResultSetColumn
	 */
	public Iterator resultSetIterator( )
	{
		PropertyHandle propHandle = getPropertyHandle( SimpleDataSet.RESULT_SET_PROP );
		assert propHandle != null;
		return propHandle.iterator( );
	}
	/**
	 * Returns an iterator for the list of computed columns. The iterator
	 * returns instances of <code>ComputedColumnHandle</code> that represents
	 * computed column object.
	 * 
	 * @return iterator over computed columns.
	 * @see org.eclipse.birt.report.model.api.elements.structures.ComputedColumn
	 */
	public Iterator computedColumnsIterator( )
	{
		PropertyHandle propHandle = getPropertyHandle( SimpleDataSet.COMPUTED_COLUMNS_PROP );
		assert propHandle != null;
		return propHandle.iterator( );
	}
	/**
	 * Returns an iterator over column hints. The iterator returns instances of
	 * <code>ColumnHintHandle</code> that represents column hint object.
	 * 
	 * @return iterator over column hints.
	 * 
	 * @see org.eclipse.birt.report.model.api.elements.structures.ColumnHint
	 */