| Oracle® OLAP Developer's Guide to the OLAP API 10g Release 1 (10.1) Part Number B10335-02 |
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View PDF |
| Oracle® OLAP Developer's Guide to the OLAP API 10g Release 1 (10.1) Part Number B10335-02 |
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|
View PDF |
This chapter describes the Oracle OLAP API Cursor class and its related classes, which you use to retrieve the results of a query. This chapter also describes the Cursor concepts of position, fetch size, and extent. For examples of creating and using a Cursor and its related objects, see Chapter 10, " Retrieving Query Results".
This chapter includes the following topics:
For the complete code of the examples in this chapter, see the example programs available from the Overview of the Oracle OLAP Java API Reference.
A Cursor retrieves the result set defined by a Source. You can also get the SQL generated for a Source by the Oracle OLAP SQL generator without having to create a Cursor.
To get the SQL for the Source, you create an ExpressSQLCursorManager by using the createSQLCursorManager method of a DataProvider. You can then use classes outside of the OLAP API to retrieve data using the generated SQL.
The Oracle OLAP API has two paths to the creation of a Cursor for a Source. The older method requires creating a CursorManagerSpecification, then creating a CursorManager, and then creating a Cursor. The newer method eliminates the CursorManagerSpecification. Instead, you simply create a CursorManager for the Source and then create a Cursor.
In the older method, after creating a Source that defines the data that you want to retrieve from the data store, you create a Cursor for that Source by doing the following:
Creating a CursorManagerSpecification by passing the Source to the createCursorManagerSpecification method of the DataProvider that you are using. The CursorManagerSpecification has CursorSpecification objects in a structure that mirrors the structure of the Source.
Creating a CursorManager by calling the createCursorManager method of the DataProvider and passing it the CursorManagerSpecification. The CursorManager creates Cursor objects. It also manages the local data cache for its Cursor objects and is aware of changes to the Source for a dynamic query or a parameterized Source. If the Source for the CursorManagerSpecification has inputs, then you must also pass to the createCursorManager method an array of Source objects for those inputs.
Creating a Cursor by calling the createCursor method of the CursorManager. The structure of the Cursor mirrors the structures of the CursorManagerSpecification and the Source. The CursorSpecification objects of a CursorManagerSpecification specify the behavior of their corresponding Cursor objects. If the Source for the CursorManagerSpecification has inputs, then you must also pass to the createCursor method an array of CursorInput objects that specify values for the input Source objects.
For an example of creating a Cursor using this method, see Chapter 10.
This architecture provides great flexibility in fetching data from a result set and in selecting data to display. You can do the following:
Create more than one CursorManagerSpecification object for the same Source. You can specify different behavior on the CursorSpecification components of the various CursorManagerSpecification objects in order to retrieve and display different sets of values from the same result set. You might want to do this when displaying the data from a Source in different formats, such as in a table and a crosstab.
Receive notification that the Source produced by the Template has changed. If you add a CursorManagerUpdateListener to the CursorManager for a Source, then the CursorManager notifies the CursorManagerUpdateListener when the Source for a dynamic query has changed and you that therefore need to update the CursorManagerSpecification for the CursorManager.
Update the CursorManagerSpecification for a CursorManager. If you are using Template objects to produce a dynamic query and the state of a Template changes, then the Source produced by the Template changes. If you have created a Cursor for the Source produced by the Template, then you need to replace the CursorManagerSpecification for the CursorManager with an updated CursorManagerSpecification for the changed Source. You can then create a new Cursor from the CursorManager.
Create different Cursor objects from the same CursorManager and set different fetch sizes on those Cursor objects. You might do this when you want to display the same data as a table and as a graph.
This older method of creating a CursorManager returns an ExpressSpecifiedCursorManager.
In the newer method, you create a Cursor for a Source by doing the following:
Creating a CursorManager by calling one of the createCursorManager methods of the DataProvider and passing it the Source. If you want to alter the behavior of the Cursor, then you can create a CursorInfoSpecification and use its methods to specify the behavior. You then create a CursorManager with a method that takes the Source and the CursorInfoSpecification.
Creating a Cursor by calling the createCursor method of the CursorManager.
This newer method of creating a CursorManager returns an ExpressDataCursorManager.
Some Source objects do not specify data that a Cursor can retrieve from the data store. The following are Source objects for which you cannot create a Cursor.
A Source that specifies an operation that is not computationally possible. An example is a Source that specifies an infinite recursion.
A Source that defines an infinite result set. An example is the fundamental Source that represents the set of all String objects.
A Source that has no elements or includes another Source that has no elements. Examples are a Source returned by the getEmptySource method of DataProvider and another Source derived from the empty Source. Another example is a derived Source that results from selecting a value from a primary Source that you got from an MdmDimension and the selected value does not exist in the dimension.
When you create a derived Source or change the state of a Template, you create the Source in the context of the current Transaction. The Source is active in the Transaction in which you create it or in a child Transaction of that Transaction. A Source must be active in the current Transaction for you to be able to create a Cursor for it.
Creating a derived Source occurs in a write Transaction. Creating a Cursor occurs in a read Transaction. After creating a derived Source, and before you can create a Cursor for that Source, you must change the write Transaction into a read Transaction by calling the prepareCurrentTransaction and commitCurrentTransaction methods of the TransactionProvider your application is using. For information on Transaction and TransactionProvider objects, see Chapter 8, " Using a TransactionProvider".
For a Cursor that you create for a query that includes a parameterized Source, you can change the value of the Parameter object and then get the new values of the Cursor without having to prepare and commit the Transaction again. For information on parameterized Source objects, see Chapter 6, " Understanding Source Objects".
In the oracle.olapi.data.cursor package, the Oracle OLAP API defines the interfaces described in the following table.
| Interface | Description |
|---|---|
Cursor |
An abstract superclass that encapsulates the notion of a current position. |
ValueCursor |
A Cursor that has a value at the current position. A ValueCursor has no child Cursor objects. |
CompoundCursor |
A Cursor that has child Cursor objects, which are a child ValueCursor for the values of its Source and an output child Cursor for each output of the Source. |
The structure of a Cursor mirrors the structure of its Source. If the Source does not have any outputs, then the Cursor for that Source is a ValueCursor. If the Source has one or more outputs, then the Cursor for that Source is a CompoundCursor. A CompoundCursor has as children a base ValueCursor, which has the values of the base of the Source of the CompoundCursor, and one or more output Cursor objects.
The output of a Source is another Source. An output Source can itself have outputs. The child Cursor for an output of a Source is a ValueCursor if the output Source does not have any outputs and a CompoundCursor if it does.
Example 9-1 creates a query that specifies the prices of selected product items for selected months. In the example, timeHier is a Source for a hierarchy of a dimension of time values, and prodHier is a Source for a hierarchy of a dimension of product values.
If you create a Cursor for prodSel or for timeSel, then either Cursor is a ValueCursor because both prodSel and timeSel have no outputs.
The unitPrice object is a Source for an MdmMeasure that represents values for the price of product units. The MdmMeasure has as inputs the MdmPrimaryDimension objects representing products and times, and the unitPrice Source has as inputs the Source objects for those dimensions.
The example selects elements of the dimension hierarchies and then joins the Source objects for the selections to that of the measure to produce querySource, which has prodSel and timeSel as outputs.
Example 9-1 Creating the querySource Query
Source timeSel = timeHier.selectValues(new String[]
{"CALENDAR::MONTH::55",
"CALENDAR::MONTH::58",
"CALENDAR::MONTH::61",
"CALENDAR::MONTH::64"});
Source prodSel = prodHier.selectValues(new String[]
{"PRODUCT_ROLLUP::ITEM::13",
"PRODUCT_ROLLUP::ITEM::14",
"PRODUCT_ROLLUP::ITEM::15"});
Source querySource = unitPrice.join(prodSel).join(timeSel);
The result set defined by querySource is the unit price values for the selected products for the selected months. The results are organized by the outputs. Since timeSel is joined to the Source produced by the unitPrice.join(prodSel) operation, timeSel is the slower varying output, which means that the result set specifies the set of selected products for each selected time value. For each time value the result set has three product values so the product values vary faster than the time values. The values of the base ValueCursor of querySource are the fastest varying of all, because there is one price value for each product for each day.
Example 10-1 in Chapter 10, creates a Cursor, queryCursor, for querySource. Since querySource has outputs, queryCursor is a CompoundCursor. The base ValueCursor of queryCursor has values from unitPrice, which is the base Source of the operation that created querySource. The values from unitPrice are those specified by the outputs.The outputs for queryCursor are a ValueCursor that has values from prodSel and a ValueCursor that has values from timeSel.
Figure 9-1 illustrates the structure of queryCursor. The base ValueCursor and the two output ValueCursor objects are the children of queryCursor, which is the parent CompoundCursor.
The following table displays the values from queryCursor in a table. The left column has time values, the middle column has product values, and the right column has the unit price of the product for the month.
| Month | Product | Price of Unit |
|---|---|---|
| 55 | 13 | 2426.07 |
| 55 | 14 | 3223.28 |
| 55 | 15 | 3042.22 |
| 58 | 13 | 2412.42 |
| 58 | 14 | 3107.65 |
| 58 | 15 | 3026.12 |
| 61 | 13 | 2505.57 |
| 61 | 14 | 3155.91 |
| 61 | 15 | 2892.18 |
| 64 | 13 | 2337.30 |
| 64 | 14 | 3105.53 |
| 64 | 15 | 2856.86 |
For examples of getting the values from a ValueCursor, see Chapter 10.
CursorSpecification objects specify some aspects of the behavior of their corresponding Cursor objects. You must specify the behavior on a CursorSpecification before creating the corresponding Cursor. To specify the behavior, use the following CursorSpecification methods:
setDefaultFetchSize
setExtentCalculationSpecified
setParentEndCalculationSpecified
setParentStartCalculationSpecified
specifyDefaultFetchSizeOnChildren
(for a CompoundCursorSpecification only)
A CursorSpecification also has methods that you can use to discover if the behavior is specified. Those methods are the following:
isExtentCalculationSpecified
isParentEndCalculationSpecified
isParentStartCalculationSpecified
If you have used the CursorSpecification methods to set the default fetch size, or to calculate the extent or the starting or ending positions of a value in its parent, then you can successfully use the following Cursor methods:
getExtent
getFetchSize
getParentEnd
getParentStart
setFetchSize
For examples of specifying Cursor behavior, see Chapter 10. For information on fetch sizes, see "About Fetch Sizes". For information on the extent of a Cursor, see "What is the Extent of a Cursor?". For information on the starting and ending positions in a parent Cursor of the current value of a Cursor, see "About the Parent Starting and Ending Positions in a Cursor".
A CursorManagerSpecification for a Source has one or more CursorSpecification objects. The structure of those objects reflects the structure of the Source. For example, a Source that has outputs has a top-level, or root, CursorSpecification for the Source, a child CursorSpecification for the values of the Source, and a child CursorSpecification for each output of the Source.
A Source that does not have any outputs has only one set of values. A CursorManagerSpecification for that Source therefore has only one CursorSpecification. That CursorSpecification is the root CursorSpecification of the CursorManagerSpecification.
You can create a CursorManagerSpecification for a multidimensional Source that has one or more inputs. If you do so, then you need to supply a Source for each input when you create a CursorManager for the CursorManagerSpecification. You must also supply a CursorInput for each input Source when you create a Cursor from the CursorManager. You might create a CursorManagerSpecification for a Source with inputs if you want to use a CursorManager to create a series of Cursor objects with each Cursor retrieving data specified by a different set of single values for the input Source objects.
The structure of a Cursor reflects the structure of its CursorManagerSpecification. A Cursor can be a single ValueCursor, for a Source with no outputs, or a CompoundCursor with child Cursor objects, for a Source with outputs. Each Cursor corresponds to a CursorSpecification in the CursorManagerSpecification. You use CursorSpecification methods to specify aspects of the behavior of the corresponding Cursor.
If your application uses Template objects, and a change occurs in the state of a Template so that the structure of the Source produced by the Template changes, then any CursorManagerSpecification objects that the application created for the Source expire. If a CursorManagerSpecification expires, then you must create a new CursorManagerSpecification. You can then either use the new CursorManagerSpecification to replace the old CursorManagerSpecification of a CursorManager or use it to create a new CursorManager. You can discover whether a CursorManagerSpecification has expired by calling the isExpired method of the CursorManagerSpecification.
The CursorInfoSpecification interface and its subinterfaces CompoundCursorInfoSpecification and ValueCursorInfoSpecification, specify methods for the abstract CursorSpecification class and the concrete CompoundCursorSpecification and ValueCursorSpecification classes. A CursorSpecification specifies certain aspects of the behavior of the Cursor that corresponds to it. You can create instances of classes that implement the CursorInfoSpecification interface either directly or indirectly.
You can create a CursorInfoSpecification for a Source directly by calling the createCursorInfoSpecification method of a DataProvider. You can use the methods of the CursorInfoSpecification to specify aspects of the behavior of a Cursor. You can then use the CursorInfoSpecification in creating a CursorManager by passing it as the cursorInfoSpec argument to the createCursorManager method of a DataProvider.
You can create a CursorInfoSpecification for a Source indirectly by creating a CursorManagerSpecification. You pass a Source to the createCursorManagerSpecification method of a DataProvider and the CursorManagerSpecification returned has a root CursorSpecification for that Source. If the Source has outputs, then the CursorManagerSpecification also has a child CursorSpecification for the values of the Source and one for each output of the Source.
With CursorSpecification methods, you can do the following:
Get the Source that corresponds to the CursorSpecification.
Get or set the default fetch size for the corresponding Cursor.
On a CompoundCursorSpecification, specify that the default fetch size is set on the children of the corresponding Cursor.
Specify that Oracle OLAP should calculate the extent of a Cursor.
Determine whether calculating the extent is specified.
Specify that Oracle OLAP should calculate the starting or ending position of the current value of the corresponding Cursor in its parent Cursor. If you know the starting and ending positions of a value in the parent, then you can determine how many faster varying elements the parent Cursor has for that value.
Determine whether calculating the starting or ending position of the current value of the corresponding Cursor in its parent is specified.
Accept a CursorSpecificationVisitor.
For more information, see "About Cursor Positions and Extent" and "About Fetch Sizes".
In the oracle.olapi.data.source package, the Oracle OLAP API defines the classes described in the following table.
| Interface | Description |
|---|---|
CursorInfoSpecification |
An interface that specifies methods for CursorSpecification objects. |
CursorSpecification |
An abstract class that implements some methods of the CursorInfoSpecification interface. |
CompoundCursorSpecification |
A CursorSpecification for a Source that has one or more outputs. A CompoundCursorSpecification has component child CursorSpecification objects. |
CompoundInfoCursorSpecification |
An interface that specifies methods for CompoundCursorSpecification objects. |
ValueCursorSpecification |
A CursorSpecification for a Source that has values and no outputs. |
ValueCursorInfoSpecification |
An interface for ValueCursorSpecification objects. |
A Cursor has the same structure as its CursorManagerSpecification. For every ValueCursorSpecification or CompoundCursorSpecification of a CursorManagerSpecification, a Cursor has a corresponding ValueCursor or CompoundCursor. To be able to get certain information or behavior from a Cursor, your application must specify that it wants that information or behavior by calling methods of the corresponding CursorSpecification before it creates the Cursor.
The OLAP API has the following concrete classes for creating a Cursor for a Source or for getting the SQL generated by a Source.
ExpressDataCursorManager
ExpressSpecifiedCursorManager
ExpressSQLCursorManager
An ExpressSQLCursorManager has methods that return the SQL generated by the Oracle OLAP SQL generator for the Source. You create one or more ExpressSQLCursorManager objects by calling the createSQLCursorManager or createSQLCursorManagers methods of a DataProvider. You do not use an ExpressSQLCursorManager to create a Cursor to retrieve the result set of the query specified by the Source. Instead, you use the SQL returned by the ExpressSQLCursorManager with classes outside of the OLAP API to retrieve the data specified by the query.
An ExpressDataCursorManager or ExpressSpecifiedCursorManager returned by one of the createCursorManager methods of a DataProvider manages the buffering of data for the Cursor objects it creates.
You can create more than one Cursor from the same cursor manager, which is useful for displaying data from a result set in different formats such as a table or a graph. All of the Cursor objects created by a cursor manager have the same specifications, such as the default fetch sizes. Because the Cursor objects have the same specifications, they can share the data managed by the cursor manager.
An ExpressSpecifiedCursorManager implements the SpecifiedCursorManager interface, which extends the CursorManager interface. A CursorManager has methods for creating a Cursor, for discovering whether the CursorManagerSpecification for the CursorManager needs updating, and for adding or removing a CursorManagerUpdateListener. The SpecifiedCursorManager interface adds methods for updating the CursorManagerSpecification, for discovering whether the SpecifiedCursorManager is open, and for closing it. Some of the createCursorManager methods of DataProvider return an ExpressSpecifiedCursorManager, which is an implementation of the SpecifiedCursorManager interface.
When your application no longer needs a SpecifiedCursorManager, it should close it to free resources in the application and in Oracle OLAP. To close the SpecifiedCursorManager, call its close method.
If your application is using OLAP API Template objects and the state of a Template changes in a way that alters the structure of the Source produced by the Template, then any CursorManagerSpecification objects for the Source are no longer valid. You need to create new CursorManagerSpecification objects for the changed Source.
After creating a new CursorManagerSpecification, you can create a new CursorManager for the Source. You do not, however, need to create a new CursorManager. You can call the updateSpecification method of the existing CursorManager to replace the previous CursorManagerSpecification with the new CursorManagerSpecification. You can then create a new Cursor from the CursorManager.
To determine whether the CursorManagerSpecification for a CursorManager needs updating, call the isSpecificationUpdateNeeded method of the CursorManager. You can also use a CursorManagerUpdateListener to listen for events generated by changes in a Source. For more information, see "CursorManagerUpdateListener Class" .
This topic describes CursorInput, CursorManagerUpdateListener, and CursorManagerUpdateEvent classes in the oracle.olapi.data.cursor package.
For Oracle OLAP in Oracle Database 10g, the OLAP API includes Parameter classes, which are more convenient than CursorInput objects. With a Parameter, you can create a parameterized Source. You can create a CursorManagerSpecification for a query that includes a parameterized Source, and then create a CursorManager and a Cursor.
You can then change the value of the Parameter, which changes the selection of dimension or measure elements specified by the parameterized Source. The Cursor for the query then has the new set of values for the changed query. You do not need to prepare and commit the Transaction again before getting the values of the Cursor. For information on parameterized Source objects, see Chapter 6, " Understanding Source Objects".
A CursorInput provides a value for a Source that you include in the array of Source objects that is the inputSources argument to the createCursorManager method of a DataProvider. If you create a CursorManagerSpecification for a Source that has one or more inputs, then you must provide an inputSources argument when you create a CursorManager for that CursorManagerSpecification. You include a Source in the inputSources array for each input of the Source that you pass to the createCursorManagerSpecification method.
When you create a CursorInput object, you can specify either a single value or a ValueCursor. If you specify a ValueCursor, then you can call the synchronize method of the CursorInput to make the value of the CursorInput be the current value of the ValueCursor.
CursorManagerUpdateListener is an interface that has methods that receive CursorManagerUpdateEvent object. Oracle OLAP generates a CursorManagerUpdateEvent object in response to a change that occurs in a Source that is produced by a Template or when a CursorManager updates its CursorManagerSpecification. Your application can use a CursorManagerUpdateListener to listen for events that indicate it might need to create new Cursor objects from the CursorManager or to update its display of data from a Cursor.
To use a CursorManagerUpdateListener, implement the interface, create an instance of the class, and then add the CursorManagerUpdateListener to the CursorManager for a Source. When a change to the Source occurs, the CursorManager calls the appropriate method of the CursorManagerUpdateListener and passes it a CursorManagerUpdateEvent. Your application can then perform the tasks needed to generate new Cursor objects and update the display of values from the result set that the Source defines.
You can implement more than one version of the CursorManagerUpdateListener interface. You can add instances of them to the same CursorManager.
Oracle OLAP generates a CursorManagerUpdateEvent object in response to a change that occurs in a Source that is produced by a Template or when a CursorManager updates its CursorManagerSpecification.
You do not directly create instances of this class. Oracle OLAP generates CursorManagerUpdateEvent objects and passes them to the appropriate methods of any CursorManagerUpdateListener objects you have added to a CursorManager. The CursorManagerUpdateEvent has a field that indicates the type of event that occurred. A CursorManagerUpdateEvent has methods you can use to get information about it.
A Cursor has one or more positions. The current position of a Cursor is the position that is currently active in the Cursor. To move the current position of a Cursor call the setPosition or next methods of the Cursor.
Oracle OLAP does not validate the position that you set on the Cursor until you attempt an operation on the Cursor, such as calling the getCurrentValue method. If you set the current position to a negative value or to a value that is greater than the number of positions in the Cursor and then attempt a Cursor operation, then the Cursor throws a PositionOutOfBoundsException.
The extent of a Cursor is described in "What is the Extent of a Cursor?" .
The current position of a ValueCursor specifies a value, which you can retrieve. For example, prodSel, a derived Source described in "Structure of a Cursor" , is a selection of three products from a primary Source that specifies a dimension of products and their hierarchical groupings. The ValueCursor for prodSel has three elements. The following example gets the position of each element of the ValueCursor, and displays the value at that position. The context object has a method that displays text.
// prodSelValCursor is the ValueCursor for prodSel
context.println("ValueCursor Position Value ");
context.println("-------------------- ------------------------ ");
do
{
context.println(" " + prodSelValCursor.getPosition() +
" " + prodSelValCursor.getCurrentValue());
} while(prodSelValCursor.next());
The preceding example displays the following:
ValueCursor Position Value
-------------------- ------------------------
1 PRODUCT_ROLLUP::ITEM::13
2 PRODUCT_ROLLUP::ITEM::14
3 PRODUCT_ROLLUP::ITEM::15
The following example sets the current position of prodSelValCursor to 2 and retrieves the value at that position.
prodSelValCursor.setPosition(2); context.println(prodSelValCursor.getCurrentString());
The preceding example displays the following:
PRODUCT_ROLLUP::ITEM::14
For more examples of getting the current value of a ValueCursor, see Chapter 10.
A CompoundCursor has one position for each set of the elements of its descendent ValueCursor objects. The current position of the CompoundCursor specifies one of those sets.
For example, querySource, the Source created in Example 9-1, has values from a measure, unitPrice. The values are the prices of product units at different times. The outputs of querySource are Source objects that represent selections of four month values from a time dimension and three product values from a product dimension.
The result set for querySource has one measure value for each tuple (each set of output values), so the total number of values is twelve (one value for each of the three products for each of the four months). Therefore, the queryCursor CompoundCursor created for querySource has twelve positions.
Each position of queryCursor specifies one set of positions of its outputs and its base ValueCursor. For example, position 1 of queryCursor defines the following set of positions for its outputs and its base ValueCursor:
Position 1 of output 1 (the ValueCursor for timeSel)
Position 1 of output 2 (the ValueCursor for prodSel)
Position 1 of the base ValueCursor for queryCursor (This position has the value from the unitPrice measure that is specified by the values of the outputs.)
Figure 9-2 illustrates the positions of queryCursor CompoundCursor, its base ValueCursor, and its outputs.
The ValueCursor for queryCursor has only one position because only one value of unitPrice is specified by any one set of values of the outputs. For a query like querySource, the ValueCursor of its Cursor has only one value, and therefore only one position, at a time for any one position of the root CompoundCursor.
Figure 9-3 illustrates one possible display of the data from queryCursor. It is a crosstab view with four columns and five rows. In the left column are the month values. In the top row are the product values. In each of the intersecting cells of the crosstab is the price of the product for the month.
A CompoundCursor coordinates the positions of its ValueCursor objects relative to each other. The current position of the CompoundCursor specifies the current positions of its descendent ValueCursor objects. Example 9-2 sets the position of queryCursor and then gets the current values and the positions of the child Cursor objects.
Example 9-2 Setting the CompoundCursor Position and Getting the Current Values
CompoundCursor rootCursor = (CompoundCursor) queryCursor;
ValueCursor baseValueCursor = rootCursor.getValueCursor();
List outputs = rootCursor.getOutputs();
ValueCursor output1 = (ValueCursor) outputs.get(0);
ValueCursor output2 = (ValueCursor) outputs.get(1);
int pos = 5;
rootCursor.setPosition(pos);
System.out.println("CompoundCursor position set to " + pos + ".");
System.out.println("The current position of the CompoundCursor is = "
+ rootCursor.getPosition() + ".");
System.out.println("Output 1 position = " + output1.getPosition() +
", value = " + output1.getCurrentValue());
System.out.println("Output 2 position = " + output2.getPosition() +
", value = " + output2.getCurrentValue());
System.out.println("VC position = " + baseValueCursor.getPosition() +
", value = " + baseValueCursor.getCurrentValue());
Example 9-2 displays the following:
CompoundCursor position set to 5. The current position of the CompoundCursor is 5. Output 1 position = 2, value = CALENDAR::MONTH::58 Output 2 position = 2, value = PRODUCT_ROLLUP::ITEM::14 VC position = 1, value = 3107.65
The positions of queryCursor are symmetric in that the result set for querySource always has three product values for each time value. The ValueCursor for prodSel, therefore, always has three positions for each value of the timeSel ValueCursor. The timeSel output ValueCursor is slower varying than the prodSel ValueCursor.
In an asymmetric case, however, the number of positions in a ValueCursor is not always the same relative to its slower varying output. For example, if the price of units for product 15 for month 64 were null because that product was no longer being sold by that date, and if null values were suppressed in the query, then queryCursor would only have eleven positions. The ValueCursor for prodSel would only have two positions when the position of the ValueCursor for timeSel was 4.
Example 9-3 demonstrates an asymmetric result set that is produced by selecting elements of one dimension based on a comparison of measure values. The example uses the same product and time selections as in Example 9-1. It uses a Source for a measure of product units sold, units, that is dimensioned by product, time, sales channels, and customer dimensions. The chanSel and custSel objects are selections of single values of the dimensions. The example produces a Source, querySource2, that specifies which of the selected products sold more than one unit for the selected time, channel, and customer values.
Because querySource2 is a derived Source, this example prepares and commits the current Transaction. The TransactionProvider in the example is tp. For information on Transaction objects, see Chapter 8.
The example creates a Cursor for querySource2, loops through the positions of the CompoundCursor, gets the position and current value of the first output ValueCursor and the ValueCursor of the CompoundCursor, and displays the positions and values of the ValueCursor objects. The getLocalValue method is a method in the program that extracts the local value from a unique value.
Example 9-3 Positions in an Asymmetric Query
// Create the query
querySource2 = prodSel.join(unitPrice).join(timeSel);
// Prepare and commit the current Transaction.
try
{
tp.prepareCurrentTransaction();
}
catch(NotCommittableException e)
{
output.println("Cannot commit current Transaction " + e);
}
tp.commitCurrentTransaction();
// Create the Cursor. The DataProvider is dp.
CursorManagerSpecification cursorMngrSpec =
dp.createCursorManagerSpecification(querySource2);
CursorManager cursorManager = dp.createCursorManager(cursorMngrSpec);
Cursor queryCursor2 = cursorManager.createCursor();
CompoundCursor rootCursor = (CompoundCursor) queryCursor2;
ValueCursor baseValueCursor = rootCursor.getValueCursor();
List outputs = rootCursor.getOutputs();
ValueCursor output1 = (ValueCursor) outputs.get(0);
// Get the positions and values and display them.
System.out.println("CompoundCursor Output ValueCursor" +
" ValueCursor");
System.out.println(" position position | value " +
"position | value");
do
{
System.out.println(" " + rootCursor.getPosition() +
" " + output1.getPosition() +
" " + getLocalValue(output1.getCurrentString()) +
" " + baseValueCursor.getPosition() + " "
+ getLocalValue(baseValueCursor.getCurrentString()));
}
while(queryCursor2.next());
Example 9-3 displays the following:
CompoundCursor Output ValueCursor ValueCursor
position position | value position | value
1 1 55 1 13
2 1 55 2 14
3 1 55 3 15
4 2 58 1 15
5 3 61 1 14
6 3 61 2 15
7 4 64 1 13
8 4 64 2 14
Because not every combination of product and time selections has unit sales greater than 1 for the specified channel and customer selections, the number of elements of the ValueCursor for the values derived from prodSel is not the same for each value of the output ValueCursor. For time value 55, all three products have sales greater than one, but for time value 58, only one of the products does. The other two time values, 61 and 64, have two products that meet the criteria. Therefore, the ValueCursor for the CompoundCursor has three positions for time 55, only one position for time 58, and two positions for times 61 and 64.
To effectively manage the display of the data that you get from a CompoundCursor, you sometimes need to know how many faster varying values exist for the current slower varying value. For example, suppose that you are displaying in a crosstab one row of values from an edge of a cube, then you might want to know how many columns to draw in the display for the row.
To determine how many faster varying values exist for the current value of a child Cursor, you find the starting and ending positions of that current value in the parent Cursor. Subtract the starting position from the ending position and then add 1, as in the following.
long span = (cursor.getParentEnd() - cursor.getParentStart()) + 1;
The result is the span of the current value of the child Cursor in its parent Cursor, which tells you how many values of the fastest varying child Cursor exist for the current value. Calculating the starting and ending positions is costly in time and computing resources, so you should only specify that you want those calculations performed when your application needs the information.
An Oracle OLAP API Cursor enables your application to have only the data that it is currently displaying actually present on the client computer. For information on specifying the amount of data for a Cursor, see "About Fetch Sizes".
From the data on the client computer, however, you cannot determine at what position of its parent Cursor the current value of a child Cursor begins or ends. To get that information, you use the getParentStart and getParentEnd methods of a Cursor.
For example, suppose your application has a Source named cube that represents a cube that has an asymmetric edge. The cube has four outputs. The cube Source defines products with unit sales greater than one purchased by a certain customers during three months of the year 2001. The products were sold through the direct sales channel.
You create a Cursor for that Source and call it cubeCursor. The CompoundCursor cubeCursor has the following child Cursor objects:
output 1, a ValueCursor for the channel values
output 2, a ValueCursor for the time values
output 4, a ValueCursor for the customer values
The base ValueCursor, which has values that are the products with unit sales greater than one.
Figure 9-4 illustrates the parent, cubeCursor, with the values of its child Cursor objects layered horizontally. The slowest varying output, with the channel value, is at the top and the fastest varying child, with the product values, is at the bottom. The only portion of the edge that you are currently displaying in the user interface is the block between positions 9 and 12 of cubeCursor, which is shown within the bold border. The positions, 1 through 15, of cubeCursor appear over the top row.
Figure 9-4 Values of the ValueCursor Children of cubeCursor
The current value of the output ValueCursor for the time Source is 44. You cannot determine from the data within the block that the starting and ending positions of the current value, 44, in the parent, cubeCursor, are 5 and 9, respectively.
The cubeCursor from the previous figure is shown again in Figure 9-5, this time with the range of the positions of the parent, cubeCursor, for each of the values of the child Cursor objects. By subtracting the smaller value from the larger value and adding one, you can compute the span of each value. For example, the span of the time value 44 is (9 - 5 + 1) = 5.
Figure 9-5 The Range of Positions of the Child Cursor Objects of cubeCursor
To specify that you want Oracle OLAP to calculate the starting and ending positions of a value of a child Cursor in its parent Cursor, call the setParentStartCalculationSpecified and setParentEndCalculationSpecified methods of the CursorSpecification corresponding to the Cursor. You can determine whether calculating the starting or ending positions is specified by calling the isParentStartCalculationSpecified or isParentEndCalculationSpecified methods of the CursorSpecification. For an example of specifying these calculations, see Chapter 10.
The extent of a Cursor is the total number of elements it contains relative to any slower varying outputs. Figure 9-6 illustrates the number of positions of each child Cursor of cubeCursor relative to the value of its slower varying output. The child Cursor objects are layered horizontally with the slowest varying output at the top.
The total number of elements in cubeCursor is fifteen so the extent of cubeCursor is therefore fifteen. That number is over the top row of the figure. The top row is the ValueCursor for the channel value. The extent of the ValueCursor for channel values is one because it has only one value.
The second row down is the ValueCursor for the time values. Its extent is 3, since there are 3 months values. The next row down is the ValueCursor for the customer values. The extent of its elements depends on the value of the slower varying output, which is time. The extent of the customers ValueCursor for the first month is two, for the second month it is two, and for the third month it is three.
The bottom row is the base ValueCursor for the cubeCursor CompoundCursor. Its values are products. The extent of the elements of the products ValueCursor depends on the values of the customers ValueCursor and the time ValueCursor. For example, since three products values are specified by the first set of month and customer values (products 13, 14, and 15 for customer 58 for time 43), the extent of the products ValueCursor for that set is 3. For the second set of values for customers and times (customer 61 for time 43), the extent of the products ValueCursor is 1, and so on.
Figure 9-6 The Number of Elements of the Child Cursor Objects of cubeCursor
The extent is information that you can use, for example, to display the correct number of columns or correctly-sized scroll bars. The extent, however, can be expensive to calculate. For example, a Source that represents a cube might have four outputs. Each output might have hundreds of values. If all null values and zero values of the measure for the sets of outputs are eliminated from the result set, then to calculate the extent of the CompoundCursor for the Source, Oracle OLAP must traverse the entire result space before it creates the CompoundCursor. If you do not specify that you wants the extent calculated, then Oracle OLAP only needs to traverse the sets of elements defined by the outputs of the cube as specified by the fetch size of the Cursor and as needed by your application.
To specify that you want Oracle OLAP to calculate the extent for a Cursor, call the setExtentCalculationSpecified method of the CursorSpecification corresponding to the Cursor. You can determine whether calculating the extent is specified by calling the isExtentCalculationSpecified method of the CursorSpecification. For an example of specifying the calculation of the extent of a Cursor, see Chapter 10.
An OLAP API Cursor represents the entire result set for a Source. The Cursor is a virtual Cursor, however, because it retrieves only a portion of the result set at a time from Oracle OLAP. A CursorManager manages a virtual Cursor and retrieves results from Oracle OLAP as your application needs it. By managing the virtual Cursor, the CursorManager relieves your application of a substantial burden.
The amount of data that a Cursor retrieves in a single fetch operation is determined by the fetch size specified for the Cursor. You specify a fetch size to limit the amount of data your application needs to cache on the local computer and to maximize the efficiency of the fetch by customizing it to meet the needs of your method of displaying the data.
You can also regulate the number of elements that Oracle OLAP returns by using Parameter and parameterized Source objects in constructing your query. For more information on Parameter objects, see Chapter 6, " Understanding Source Objects". For examples of using parameterized Source objects, see Chapter 7, " Making Queries Using Source Methods".
When you create a CursorManagerSpecification for a Source, as the first step in creating a Cursor, Oracle OLAP specifies a default fetch size on the root CursorSpecification of the CursorManagerSpecification. You can change the default fetch size with the setDefaultFetchSize method of the root CursorSpecification. You can also change the fetch size with the setFetchSize method of the CursorManager that you create using the CursorManagerSpecification, or with the setFetchSize method of a Cursor that you create with the CursorManager.
You can create two or more Cursor objects from the same CursorManager and use both Cursor objects simultaneously. Rather than having separate data caches, the Cursor objects can share the data managed by the CursorManager.
An example is an application that displays the results of a query to the user as both a table and a graph. The application creates a CursorManagerSpecification for a Source and then creates a CursorManager for the CursorManagerSpecification. The application creates two separate Cursor objects from the same CursorManager, one for a table view and one for a graph view. The two views share the same query and display the same data, just in different formats. Figure 9-7 illustrates the relationship between the Source, the Cursor objects, and the views.
Figure 9-7 A Source and Two Cursors for Different Views of Its Values
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