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Using Hints to Influence the Execution Plan
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The assumption that underlies the use of the CBO is that the optimizer knows best. That is, by evaluating the various statistics, the CBO will come to the best decision in terms of choosing the optimal execution plan. However, the optimizer is based on rules, and a good application developer has knowledge about the application and data that the CBO can t exploit. You can provide hints to the optimizer to override the CBO s execution plans. For example, if you know that a certain index is more selective than another, you can force Oracle to use that index by providing the hint in your query. Hints can alter the join method, join order, or access path. You can also provide hints to parallelize the SQL statement operations. The following are some of the common hints that you can use in SQL statements: ALL_ROWS: The ALL_ROWS hint instructs Oracle to optimize throughput (that is, minimize total cost), not optimize the response time of the statement. FIRST_ROWS(n): The FIRST_ROWS(n) hint dictates that Oracle return the first n rows quickly. Low response time is the goal of this hint.
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When you specify ALL_ROWS or the FIRST_ROWS(n) hint, it overrides the current value of the OPTIMIZER_MODE parameter, if it s different from that specified by the hint.
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FULL: The FULL hint requires that a full scan be done on the table, ignoring any indexes that may be present. You would want to do this when you have reason to believe that using an index in this case will be inefficient compared to a full table scan. To force Oracle to do a full table scan, you use the FULL hint. ORDERED: This hint forces the join order for the tables in the query. INDEX: This hint forces the use of an index scan, even if the optimizer was going to ignore the indexes and do a full table scan for some reason.
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CHAPTER 19 IMPROV IN G DA TA BAS E PERFORMAN CE: SQL QUERY OPTIMIZA TION
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INDEX_FFS: An index fast full scan (INDEX_FFS) hint forces a fast full scan of an index, just as if you did a full table scan that scans several blocks at a time. INDEX_FFS scans all the blocks in an index using multiblock I/O, the size of which is determined by the DB_FILE_MULTIBLOCK_ READ_COUNT parameter. You can also parallelize an INDEX_FFS hint, and it s generally preferable to a full table scan. The OPTIMIZER_MODE settings determine the way the query optimizer performs optimization throughout the database. However, at times, due to lack of accurate statistics, the optimizer can be mistaken in its estimates, leading to poor execution plans. In cases such as this, you can use optimizer hints to override this database optimization setting at the individual SQL statement level. Oracle Database 11g also provides the SQL Profile feature. This feature enables you to collect auxiliary information using sampling and partial execution techniques, thereby avoiding the use of optimizer hints. I discuss SQL profiles in the section titled Using the SQL Tuning Advisor on SQL Statements, later in this chapter.
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Selecting the Best Join Method
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Choose a join method based on how many rows you expect to be returned from the join. The optimizer generally tries to choose the ideal join condition, but it may not do so for various reasons. It s up to you to see what join method the optimizer will adopt and change it if necessary. The following guidelines will help you when you re analyzing output produced by an EXPLAIN PLAN.
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Avoiding Cartesian Joins
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Cartesian joins usually aren t the result of intentional planning; rather, they happen due to logical mistakes in the query. Cartesian joins are produced when your joins don t have any WHERE clauses. If you re joining several tables, make sure that each table in the join is referenced by a WHERE condition. Even if the tables being joined are small, avoid Cartesian joins because they re inefficient. For example, if the employee table has 2,000 rows and the dept table has 100 rows, a Cartesian join of employee and dept will have 2,000 * 100 = 200,000 rows.
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