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Accessing a table by ROWID retrieves rows using unique ROWIDs. ROWIDs in Oracle specify the exact location in the datafile and the data block where the row resides, so ROWID access is the fastest way to retrieve a row in Oracle. Often, Oracle obtains the ROWID through an index scan of the table s indexes. Using these ROWIDs, Oracle swiftly fetches the rows.
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An index stores two things: the column value of the column on which the index is based and the ROWID of the rows in the table that contain that column value. An index scan retrieves data from an index using the values of the index columns. If the query requests only the indexed column values, Oracle will return those values. If the query requests other columns outside the indexed column, Oracle will use the ROWIDs to get the rows of the table.
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When you need to access data that s in two or more tables, Oracle joins the tables based on a common column. However, there are several ways to join the row sets returned from the execution plan steps. For each statement, Oracle evaluates the best join method based on the statistics and the type of unique or primary keys on the tables. After Oracle has evaluated the join methods, the CBO picks the join method with the least cost. The following are the common join methods used by the CBO: Nested loop join: A nested loop join involves the designation of one table as the driving table (also called the outer table) in the join loop. The other table in the join is called the inner table. Oracle fetches all the rows of the inner table for every row in the driving table. Hash join: When you join two tables, Oracle uses the smaller table to build a hash table on the join key. Oracle then searches the larger table and returns the joined rows from the hash table. Sort-merge join: The sort join operation sorts the inputs on the join key, and the merge join operation merges the sorted lists. If the input is already sorted by the join column, there s no need for a sort join operation for that row source.
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CHAPTER 19 IM PR OVING DA TA BAS E PERFORM ANC E: S QL QUE RY OPTIMIZA TION
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Choosing the Join Order
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Once the optimizer chooses the join method, it determines the order in which the tables are joined. The goal of the optimizer is always to join tables in such a way that the driving table eliminates the largest number of rows. A query with four tables has a maximum of 4 factorial, or 24, possible ways in which the tables can be joined. Each such join order would lead to a number of different execution plans, based on the available indexes and the access methods. The search for an optimal join strategy could take a long time in a query with a large number of tables, so Oracle depends on an adaptive search strategy to limit the time it takes to find the best execution plan. An adaptive search strategy means that the time taken for optimization is always a small percentage of the total time that is taken for execution of the query itself.
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Drawbacks of the CBO
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The CBO is systematic, but the optimizer is not guaranteed to follow the same plan in similar cases. However, the CBO isn t always perfect, and you need to watch out for the following: The CBO isn t fixed across Oracle versions. Execution plans can change over time as versions change. Later in this chapter, you ll see how to use stored outlines so the optimizer always uses a known plan to maintain plan stability. Application developers may know more than the CBO when it comes to choosing the best access path. Application developers know the needs of the users, of which the CBO is completely unaware. This could lead to a situation where the CBO may be optimizing throughput, when the users would rather have a quick set of results on their screen. By using hints such as FIRST_ROWS_n, you can overcome this drawback in the CBO. The CBO depends enormously on correct statistics gathering. If the statistics are absent or outdated, the optimizer can make poor decisions.
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