Bug 180189 Support in memory data set cache.
Currently, Design-Time Data Cache is stored on disks. By performance tests using ramdisk, we found that caching in physical memory can improve the performance of RunAndRender by 30%.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/PreparedDataSourceQuery.java
{
		return this;
	}
	
	/**
	 * Executes the prepared execution plan.  This returns
	 * a QueryResult object at a state ready to return its 
	 * current result iterator, or evaluate an aggregate expression.
	 * <p>
	 * This includes setup runtime state, and evaluation of
	 * any beforeOpen and afterOpen scripts on a data set. 
	 * @return The QueryResults object opened and ready to return
	 * 		the results of a report query. 
	 */
	public IQueryResults execute( Scriptable scope ) throws DataException
	{
		return this.execute( null, scope );
	}
	/**
	 * Executes the prepared execution plan as an inner query that appears
	 * within the scope of another report query. The outer query must have been
	 * prepared and executed, and its results given as a parameter to this
	 * method.
	 * 
	 * @param outerResults
	 *            QueryResults for the executed outer query
	 * @return The QueryResults object opened and ready to return the results of
	 *         a report query.
	 */
	public IQueryResults execute( IQueryResults outerResults, Scriptable scope )