Specifying the Behavior of a Cursor

About specifying the behavior of a Cursor

You can specify the following aspects of the behavior of a Cursor.

• The fetch size of a Cursor, which is the number of elements of the result set that the Cursor retrieves during one fetch operation.

• The shape of the fetch block of a Cursor. The fetch block is the set of elements of each descendent ValueCursor that the parent CompoundCursor retrieves. The shape of the fetch block is the levels of the CompoundCursor at which you set the fetch sizes.

• Whether the OLAP service calculates the extent of the Cursor. The extent is the total number of positions of the Cursor. If the Cursor is a child Cursor of a CompoundCursor, its extent is relative to any slower varying outputs.

• Whether the OLAP service calculates the positions in the parent Cursor at which the value of a child Cursor starts or ends.

To specify the behavior of Cursor, you use methods on the CursorSpecification for that Cursor. To get the CursorSpecification for a Cursor, you use methods on the CursorManagerSpecification that you create for a Source.

Note: Specifying the calculation of the extent or the starting or ending position in a parent Cursor of the current value of a child Cursor can be a very expensive operation. The calculation can require considerable time and computing resources. You should only specify these calculations when your application needs them.

For more information on the relationships of Source, Cursor, CursorSpecification, and CursorManagerSpecification objects or the concepts of fetch size, extent, or Cursor positions, see Chapter 10.

Example: Getting CursorSpecification objects from a CursorManagerSpecification

This example creates a Source, creates a CursorManagerSpecification for the Source, and then gets the CursorSpecification objects from a CursorManagerSpecification. The root CursorSpecification is the CursorSpecification for the top-level CompoundCursor.

Source salesAmountsForSelections = salesAmount.join(customerSel)
                                              .join(productSel);
                                              .join(timeSel);
                                              .join(channelSel);
                                              .join(promotionSel);
try{
  tp.prepareCurrentTransaction();
}
catch(NotCommittableException e){
  output.println("Caught exception " + e + ".");
}
tp.commitCurrentTransaction();

// Create a Cursor for salesAmountsForSelections
CursorManagerSpecification cursorMngrSpec =
     dp.createCursorManagerSpecification(salesAmountsForSelections);

// Get the root CursorSpecification of the CursorManagerSpecification.
CompoundCursorSpecification rootCursorSpec =
(CompoundCursorSpecification) cursorMngrSpec.getRootCursorSpecification();

// Get the CursorSpecification for the base values
ValueCursorSpecification baseValueSpec =
                        rootCursorSpec.getValueCursorSpecification();

// Get the CursorSpecification objects for the outputs
List outputSpecs = rootCursorSpec.getOutputs();
ValueCursorSpecification promoSelValCSpec = 
                       (ValueCursorSpecification) outputSpecs.get(0);
ValueCursorSpecification chanSelValCSpec = 
                       (ValueCursorSpecification) outputSpecs.get(1);
ValueCursorSpecification timeSelValCSpec = 
                       (ValueCursorSpecification) outputSpecs.get(2);
ValueCursorSpecification prodSelValCSpec = 
                       (ValueCursorSpecification) outputSpecs.get(3);
ValueCursorSpecification custSelValCSpec = 
                       (ValueCursorSpecification) outputSpecs.get(4);

Once you have the CursorSpecification objects, you can use their methods to specify the behavior of the Cursor objects that correspond to them.