Bug 211839 String grouping unit type needs to support interval range.
Build ID: I20071101-2000 As Chart has supported, the String grouping unit type can be set interval range, the interval range means that the string group will base on row count distinction. For instance, let's say we have a,b,c,d,e,f,g , and set interval range to 2, the grouping result should be a,c,e,g. Now it is a requirement that DtE support the grouping type.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/odi/IQuery.java
* via an external data source or cached data. 
 * The IDataSource is the factory for all types of IQuery instances. 
 * There may be many IQuery instances associated with an IDataSource. 
 * Multiple independent queries might be executed simultaneously by different threads, 
 * but the implementation might choose to execute them serially. 
 * In either case, the implementation must be thread safe. 
 * <p>
 * Interface methods are provided to bind data manipulation 
 * specification, such as ordering, aggregates/grouping, filtering.
 * <br>
 * In a later release, this query would also accept specification
 * for computed columns to let an ODI Executor, if capable, 
 * to compute the value for such column.
 * <p>
 * The execute methods run a prepared data source query with its 
 * parameter values, if applicable,
 * and return the result in a result iterator which the user can 
 * iterate to get its IResultObject instances.
 * <br>
 * An implementation might choose to represent its result iterator
 * as a cursored result supporting incremental fetch. 
 * <br>
 * The result may be a large set, which could be iterated or passed
 * to another query (ICandidateQuery) for further data manipulation. 
 */
public interface IQuery
{
	/**
	 * Bind the ordering/sorting specification to the query instance.  
	 * Specify the ordering of one or more fields in the query result objects.