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You can improve the response times of frequently executed SQL queries by using the result cache. The result cache stores results of SQL queries and PL/SQL functions in a new component of the SGA called the Result Cache Memory. The first time a repeatable query executes, the database caches its results. On subsequent executions, the database simply fetches the results from the result cache instead of executing the query again. The database manages the result cache. You can turn result caching on only at the database level. If any of the objects that are part of a query are modified, the database invalidates the cached query results. Ideal candidates for result caching are queries that access many rows to return a few rows, as in many data warehousing solutions. The result cache consists of two components, the SQL Query Result Cache that stores SQL query results and the PL/SQL Function Result Cache that stores the values returned by PL/SQL functions, with both components sharing the same infrastructure. I discuss the two components of the result cache in the following sections.

The purpose of a push interaction is for one party to deliver information and/or execution control to another. I ll use the word talker to designate the party starting the interaction, and the word listener for the party with whom the talker interacts. In a push interaction, the talker sends a command to the listener, as shown in Figure 9-1.

The result cache is always enabled by default, and its size depends on the memory the database allocates to the shared pool. If you specify the MEMORY_TARGET parameter for allocating memory, Oracle allocates 0.25% of the MEMORY_TARGET parameter value to the result cache. If you specify the SGA_TARGET parameter instead, Oracle allocates 0.5% of the SGA_TARGET value to the result cache. You can change the memory allocated to the result cache by setting the RESULT_CACHE_MAX_SIZE initialization parameter. This parameter can range from a value of zero to a system-dependent maximum. You disable result caching by setting the parameter to zero, as shown here:

SQL> ALTER SYSTEM SET result_cache_max_size=0; Since result caching is enabled by default, it means that the RESULT_CACHE_MAX_SIZE parameter has a positive default value as well, based on the size of the MEMORY_TARGET parameter (or the SGA_TARGET parameter if you have that parameter instead). In addition to the RESULT_CACHE_MAX_SIZE parameter, two other initialization parameters have a bearing on the functioning of the result cache: the RESULT_CACHE_MAX_RESULT parameter specifies the maximum amount of the result cache a single result can use. By default, a single cached result can occupy up to 5 percent of the result cache, and you can specify a percentage between 1 and 100. The RESULT_CACHE_REMOTE_EXPIRATION parameter determines the length of time for which a cached result that depends on remote objects is valid. By default, this parameter is set to zero, meaning you aren t supposed to use the result cache for queries involving remote objects. The reason for this is over time remote objects could be modified, leading to invalid results in the cache.

Whether the database caches a query result or not depends on the value of the RESULT_CACHE_MODE initialization parameter, which can take two values: MANUAL or FORCE. Here s how the two values affect result caching behavior in the database: If you set the parameter to FORCE, the database will try to use the cache for all results, wherever it s possible to do so. You can, however, skip the cache by specifying NO_RESULT_CACHE hint within a query. If you set the parameter to MANUAL, the database caches the results of a query only if you include the RESULT_CACHE hint in the query. By default, the RESULT_CACHE_MODE parameter is set to MANUAL and you can change the value dynamically as shown here: SQL> alter session set result_cache_mode=force scope=spfile;

Figure 9-1. The push-interaction pattern A command can include data. A push interaction is an imperative interaction. Push interactions are effective when the following two things apply: 1. The talker acts as a controller of the listener. 2. The talker decides when to issue commands to the listener. Push interactions form the basis of event-driven systems. Publishers are talkers, and subscribers are listeners. When something occurs to the publisher, notifications are sent to available subscribers to inform them of the event. Using push interactions to propagate changes through a system is particularly efficient when the talker needs to send the same command to a large number of listeners. If you have to implement the system without using push interactions, the alternative would require each listener to poll the talker periodically, waiting for changes to occur. A significant amount of processing time would be wasted in polling loops. Using a push interaction pattern frees the listeners from devoting resources to monitor the talker. While the talker has no new data, the listener can use its own resources to do more useful things than sitting in a polling loop. Due to their efficiency, push interactions are a good solution in real-time systems. When changes occur at very high speed, you might use a single notification to describe multiple changes at once. There are numerous examples of the push pattern in everyday life and in computing.