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During this phase, Oracle transforms the logical query plan into a physical query plan. The optimizer may be faced with a choice of several algorithms to resolve a query. It needs to choose the most efficient algorithm to answer a query, and it needs to determine the most efficient way to implement the operations. In addition to deciding on the best operational steps, the optimizer determines the order in which it will perform these steps. For example, the optimizer may decide that a join between table A and table B is called for. It then needs to decide on the type of join and the order in which it will perform the table join. The physical query or execution plan takes into account the following factors: The various operations (for example, joins) to be performed during the query The order in which the operations are performed The algorithm to be used for performing each operation The best way to retrieve data from disk or memory The best way to pass data from one operation to another during the query The optimizer may generate several valid physical query plans, all of which are potential execution plans. The optimizer then chooses among them by estimating the cost of each possible physical plan based on the table and index statistics available to it, and selecting the plan with the lowest estimated cost. This evaluation of the possible physical query plans is called cost-based query optimization. The cost of executing a plan is directly proportional to the amount of resources such as I/O, memory, and CPU necessary to execute the proposed plan. The optimizer passes this low-cost physical query plan to Oracle s query execution engine. The next section presents a simple example to help you understand the principles of cost-based query optimization.

Listing 8-12. VB .NET Example of Locking a Resource and Then Firing an Event ' DON'T DO THIS! Public Class BadEventSource Public Delegate Sub SomethingHappenedHandler() Public OnSomethingHappened As SomethingHappenedHandler Public Sub M1() SyncLock (Me) If Not OnSomethingHappened Is Nothing Then OnSomethingHappened() End If End SyncLock End Sub End Class The code lock could cause a deadlock if a SomethingHappened subscriber caused BadEventSource.M1 to be called directly or indirectly while processing the event notification. Another type of deadlock can occur even in the absence of reentrant calls, when the event source locks a resource that is shared with other objects or components. For example, say S is a file, A is an event source, and B is an event subscriber. Assume A locks S for exclusive access and then fires an event to B using a synchronous call. When B gets the event notification, say it needs access to S before it can complete its job and return control to A. Obviously, the system will freeze, with B waiting forever for A to release the lock on S. Again, the solution is to design A so it doesn t need to lock S while it fires events.

Let s say you want to run the following query, which seeks to find all the supervisors who work in Dallas. The query looks like this: SQL> SELECT * FROM employee e, dept d WHERE e.dept_no = d.dept_no AND(e.job = 'SUPERVISOR' AND d.city = 'DALLAS'); SQL> Now, you have several ways to arrive at the list of the supervisors. Let s consider three ways to arrive at this list, and compute the cost of accessing the results in each of the three ways.

Make the following simplifying assumptions for the cost computations: You can only read and write data one row at a time (in the real world, you do I/O at the block level, not the row level). The database writes each intermediate step to disk (again, this may not be the case in the real world). No indexes are on the tables. The employee table has 2,000 rows. The dept table has 40 rows. The number of supervisors is also 40 (one for each department). Ten departments are in the city of Dallas. In the following sections, you ll see three different queries that retrieve the same data, but that use different access methods. For each query, a crude cost is calculated, so you can compare how the three queries stack up in terms of resource cost. The first query uses a Cartesian join.

When an event is fired using a procedure call, there is the potential for event targets to throw exceptions while handling the event, disrupting the normal sequence of notifications. Event sources may need to be coded defensively and be prepared to handle exceptions thrown by event targets. You must consider two kinds of problems: 1. What to do if an exception occurs 2. How to proceed For the former problem, you might need to fire a different event or call a fallback method. For the latter problem, options include doing nothing, letting the exception propagate up the stack, throwing an exception of a new type, notifying the operator, and logging the exception.