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As I mentioned earlier in this chapter, the CBO doesn t always use the same execution strategies. Changes in Oracle versions or changes in the initialization parameters concerning memory allocation may force the CBO to modify its plans. You can use Oracle s plan stability feature to ensure that the execution plan remains stable regardless of any changes in the database environment. The plan stability feature uses stored outlines to preserve the current execution plans, even if the statistics and optimizer mode are changed. The CBO uses the same execution plan with identical access paths each time you execute the same query. The catch is that the query must be exactly identical each time if you want Oracle to use the stored plan.

The notifications sent in a one-to-one relationship are called unicast notifications. The signal diagram in Figure 8-20 shows a unicast notification wired from A to B.

When you use stored outlines to preserve a currently efficient execution plan, you re limiting Oracle s capability to modify its execution plans dynamically based on changes to the database environment and changes to the statistics. Ensure you use this feature for valid purposes, such as maintaining similar plans for distributed applications.

On the face of it, the stored outline feature doesn t seem impressive. Let s consider a simple example to see how a stored outline could be useful in a real production environment. Suppose you have a system that s running satisfactorily and, due to a special need, you add an index to a table. The addition of the new index could unwittingly modify the execution plans of the CBO, and your previously fast-running SQL queries may slow down. It could conceivably take a lot of effort, testing, and time to fix the problem by changing the original query. However, if you had created stored outlines, these kinds of problems wouldn t arise. Once Oracle creates an outline, it stores it until you remove it. In the next section you ll examine how to implement planned stability in a database.

Outlines are useful when you re planning migrations from one version of Oracle to another. The CBO could behave differently between versions, and you can cut your risk down by using stored outlines to preserve the application s present performance. You can also use them when you re upgrading your applications. Outlines ensure that the execution paths the queries used in a test instance successfully carry over to the production instance.

Figure 8-20. A unicast notification When firing a synchronous unicast event using procedure calls, the event handler in B is run on A s calling thread. If the handler throws an exception, A may be adversely affected. If A is expected to fire events to other event targets, those targets may not get notified, causing the system to behave incorrectly. The obvious solution is for event sources to be prepared to catch exceptions thrown by event handlers.

Stored outlines are especially useful when the users of an application have information about the environment that the Oracle CBO doesn t possess. By enabling the direct editing of stored outlines, Oracle lets you tune SQL queries without changing the underlying application. This is especially useful when you re dealing with packaged applications where you can t get at the source code.

Implementing plan stability is a simple matter. You have to ensure that the following initialization parameters are consistent in all the environments. You must set the value of the first two parameters to TRUE. The default value for OPTIMIZER_FEATURES_ENABLE is 11.1.0.6, and if you change it, make sure it s the same in all environments. Here are the relevant initialization parameters: QUERY_REWRITE_ENABLED STAR_TRANSFORMATION_ENABLED OPTIMIZER_FEATURES_ENABLE

Figure 8-21. A multicast notification Any number of subscribers may be present. When using procedure calls to deliver notifications, the sender maintains a list of subscribers and calls them in some order. In peer-to-peer (P2P) systems, one peer can subscribe to the events of another peer. An event fired by one peer may be multicast to others recursively, resulting in a cascaded multicast, as shown in Figure 8-22. In P2P systems, events are typically fired asynchronously using messages, so P1 would not block while the notification rippled through the rest of the system. When sending notifications as messages, multicasting can also be handled independently at the communication substrate level, again saving the event source from having to block while subscribers are notified. Multicasts are handled this way in an IP network, where IP routers assume the task of multicasting. The original sender in this case is oblivious to the fact that multiple subscribers are receiving its notifications.