Bug 180842 Config memory buffer size in data engine execution.
To tuning the performance, it is necessary for DtE to expose API for clients to config the size of memory it used to store the final result set. A new AppContext string, "org.eclipse.birt.data.cache.memory.size" is proposed. The new appcontext entry only accept positive integer as input, the unit of which would be MB.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/QueryExecutor.java
}
	/**
	 * Provide the actual DataSourceRuntime used for the query.
	 * 
	 * @return
	 */
	abstract protected DataSourceRuntime findDataSource( ) throws DataException;
	/**
	 * Create a new instance of data set runtime
	 * 
	 * @return
	 */
	abstract protected DataSetRuntime newDataSetRuntime( ) throws DataException;
	/**
	 * Create a new unopened odiDataSource given the data source runtime
	 * definition
	 * 
	 * @return
	 */
	abstract protected IDataSource createOdiDataSource( ) throws DataException;
	/**
	 * Create an empty instance of odi query
	 * 
	 * @return
	 */
	abstract protected IQuery createOdiQuery( ) throws DataException;
	/**
	 * Prepares the ODI query
	 */
	protected void prepareOdiQuery( ) throws DataException