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he query tree class shown in 10 forms the starting point for building the experimental query optimization and execution engine for DBXP. In this chapter, I ll show you how to add the optimizer to the query tree class. I ll begin by explaining the rationale for the heuristics (or rules) used in the optimizer, and then jump into writing the code. Because the code for some of the functions is quite lengthy, the code examples shown in this chapter are excerpts. If you are following along by coding the examples, I recommend you download the source code for this chapter instead of typing in the code from scratch.

In most cases, the use of System.Transactions will provide the best trade-off between performance and simplicity of data access code.

The first query optimizers were designed for use in early database systems such as System R1 and INGRES.2 These optimizers were developed for a particular implementation of the relational model and have stood the test of time as illustrations for how to implement optimizers. Many of the commercially available database systems are based on these works. Since then, optimizers have been created for extensions of the relational model to include object-oriented and distributed database systems. One example is the Volcano optimizer, which uses a dynamic programming algorithm to generate query plans for cost-based optimization in object-oriented database systems. Another example is concerned with how to perform optimization in heterogeneous database systems (similar to distributed systems, but there is no commonly shared concept of organization). In these environments it is possible to use statistical methods for deriving optimization strategies. Another area in which the requirements for query optimization generate unique needs is that of memory-resident database systems. Memory-resident database systems are designed to contain the entire system and all of the data in the computer s secondary memory (i.e., disk). While most of these applications are in the area of embedded systems, some larger distributed systems that consist of a collection of systems use memory-resident databases to expedite information flow. Optimization in memory-resident database systems requires faster algorithms

However, we need to create a real Cocoa NSMutableDictionary object, so that s what we ve done. Nicely, we can now go on and treat that just like a Python dictionary:

While child objects are deleted within the context of the root object that s being updated, deletion of root objects is a bit different. Recall that the data portal was implemented to support both immediate and deferred deletion of a root object.

1. P G. Selinger, M. M. Astraham, D. D. Chamberlin, R. A. Lories, and T. G. Price. 1979. Access Path Selection in . a Relational Database Management System. Proceedings of the ACM SIGMOD International Conference on the Management of Data, Aberdeen, Scotland: 23 34. Considered by some to be the Bible of Query Optimization. 2. M. Stonebraker, E. Wong, P Kreps. 1976. The Design and Implementation of INGRES. ACM Transactions on . Database Systems 1(3): 189 222.

Immediate deletion occurs when the UI code calls a static delete method on the business class, providing parameters that define the object to be deleted: typically, the same criteria that would be used to retrieve the object. Most applications will use immediate deletion for root objects. The sequence of events flows like this: 1. The static delete method is called. 2. The static delete method calls DataPortal.Delete(). 3. DataPortal.Delete() creates the business object using reflection. 4. DataPortal.Delete() calls the DataPortal_Delete() method on the business object, which contains the code needed to delete the object s data (and any related child data, and so on). 5. From the business object s perspective, two methods are called, as follows: The default constructor DataPortal_Delete() Figure 7-8 illustrates the process of immediate deletion.

Since this causes the deletion of a root object, the delete process must also remove any data for child objects. This can be done through ADO.NET data access code, through a stored procedure, or by the database (if cascading deletes are set up on the relationships). In the example application, child data is deleted by the stored procedures created in 6.