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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.
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Query Optimization and the Oracle CBO
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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.
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Choosing Your Optimization Mode
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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.
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Providing Statistics to the Optimizer
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By default, the database itself automatically collects the necessary optimizer statistics. Every night, the database schedules a statistics collection job during the maintenance window of the Oracle Scheduler. The maintenance window, by default, extends from 10 p.m. to 6 a.m. on weekdays and all weekend as well. The job is named GATHER_STATS_JOB and runs by default in every Oracle Database 11g database. You have the ability to disable the GATHER_STATS_JOB if you wish. You can get details about this default GATHER_STATS_JOB by querying the DBA_SCHEDULER_JOBS view. The GATHER_STATS_JOB collects statistics for all tables that either don t have optimizer statistics or have stale (outdated) statistics. Oracle considers an object s statistics stale if more than 10 percent of its data has changed since the last time it collected statistics for that object. By default, Oracle monitors all DML changes such as inserts, updates, and deletes made to all database objects. You can also view the information about these changes in the DBA_TAB_MODIFICATIONS view. Based on this default object monitoring, Oracle decides whether to collect new statistics for an object. To check that the GATHER_STATS_JOB is indeed collecting statistics on a regular basis, use the following: SQL> SELECT last_analyzed, table_name, owner, num_rows, sample_size 2 FROM dba_tables 3* ORDER by last_analyzed;
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