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During the final stage of query processing, the optimized query (the physical query plan that has been selected) is executed. If it s a SELECT statement, the rows are returned to the user. If it s an INSERT, UPDATE, or DELETE statement, the rows are modified. The SQL execution engine takes the execution plan provided by the optimization phase and executes it. Of the three steps involved in SQL statement processing, the optimization process is the crucial one because it determines the all-important question of how fast your data will be retrieved. Understanding how the optimizer works is at the heart of query optimization. It s important to know what the common access methods, join methods, and join orders are in order to write efficient SQL. The next section presents a detailed discussion of the all-powerful Oracle CBO.

Shared memory has long been used in distributed systems to allow different processes to exchange information. What is noteworthy is the fact that, from the perspective of an event-based programmer, information that passes through shared memory looks just like an event notification. Why For the following reasons:

In most cases, you have multiple ways to execute a SQL query. You can get the same results from doing a full table scan or using an index. You can also retrieve the same data by accessing the tables and indexes in a different order. The job of the optimizer is to find the optimal or best plan to execute your DML statements such as SELECT, INSERT, UPDATE, and DELETE. Oracle uses the CBO to help determine efficient methods to execute queries. The CBO uses statistics on tables and indexes, the order of tables and columns in the SQL statements, available indexes, and any user-supplied access hints to pick the most efficient way to access them. The most efficient way, according to the CBO, is the least costly access method, cost being defined in terms of the I/O and the CPU expended in retrieving the rows. Accessing the necessary rows means Oracle reads the database blocks on the file system into the buffer pool. The resulting I/O cost is the most expensive part of SQL statement execution because it involves reading from the disk. You can examine these access paths by using tools such as the EXPLAIN PLAN. The following sections cover the tasks you need to perform to ensure that the optimizer functions efficiently.

In older versions of Oracle, you had a choice between a rule-based and a Cost-Based Optimizer. In a rule-based approach, Oracle used a heuristic method to select among several alternative access paths with the help of certain rules. All the access paths were assigned a rank, and the path with the lowest rank was chosen. The operations with a lower rank usually executed faster than those with a higher rank. For example, a query that uses the ROWID to search for a row has a cost of 1. This is expected because identifying a row with the help of the ROWID, an Oracle pointer-like mechanism, is the fastest way to locate a row. On the other hand, a query that uses a full table scan has a cost of 19, the highest possible cost under rule-based optimization. The CBO method almost always performs better than the older rule-based approach because, among other things, it takes into account the latest statistics about the database objects.

The sender is like an event source, because it sends out information without knowing what potential receivers will do with it. Receivers look like subscribers, because they monitor the shared memory, waiting for information to appear. Sender and receiver must agree on a designated rendezvous area in the shared memory. When the notification signal is written to the rendezvous area, the subscriber must somehow clear it after handling it; otherwise, the subscriber will go into an infinite loop and keep trying to handle the notification over and over again. If the rendezvous area is a single byte, the clearing operation might entail setting the byte to 0. If the area is a data structure, you could designate a separate byte to act as the signaling flag. You should protect the flag by a semaphore or other process synchronization mechanism to prevent concurrent writing by the sender and subscriber. Figure 8-14 shows a possible solution supporting unicast events.