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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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Tip Note that the optimizer mode you choose applies only to SQL statements that are issued directly. If you use an ALTER SESSION statement to change the optimizer mode for SQL that s part of a PL/SQL code block, it ll be ignored. You must use optimizer hints, which I discuss in the section titled Using Hints to Influence the Execution Plan, to set the optimizer mode for any SQL statement that s part of a PL/SQL block.
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To determine the current optimizer mode for your database, you can run the following query: SQL> SELECT name, value FROM V$PARAMETER 2 WHERE name = 'optimizer_mode'; NAME --------------optimizer_mode SQL> VALUE -------ALL_ROWS
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Any SQL statement can override the instance- or session-level settings with the use of optimizer hints, which are directives to the optimizer for choosing the optimal access method. By using hints, you can override the instance-wide setting of the OPTIMIZER_MODE initialization parameter. See the section Using Hints to Influence the Execution Plan later in this chapter for an explanation of optimizer hints.
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What Does the Optimizer Do
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The CBO performs several intricate steps to arrive at the optimal execution plan for a user s query. The original SQL statement is most likely transformed, and the CBO evaluates alternative access paths (for example, full-table or index-based scans). If table joins are necessary, the optimizer evaluates all possible join methods and join orders. The optimizer evaluates all the possibilities and arrives at the execution plan it deems the cheapest in terms of total cost, which includes both I/O and CPU resource usage cost.
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SQL Transformation
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Oracle hardly ever executes your query in its original form. If the CBO determines that a different SQL formulation will achieve the same results more efficiently, it transforms the statement before executing it. A good example is where you submit a query with an OR condition, and the CBO transforms it into a statement using UNION or UNION ALL. Or your statement may include an index hint, but the CBO might transform the statement so it can do a full table scan, which can be more efficient under some circumstances. In any case, it s good to remember that the query a user wishes to be executed may not be executed in the same form by Oracle, but the query s results are still the same. Here are some common transformations performed by the Oracle CBO: Transform IN into OR statements. Transform OR into UNION or UNION ALL statements. Transform noncorrelated nested selects into more efficient joins. Transform outer joins into more efficient inner joins. Transform complex subqueries into joins, semijoins, and antijoins. Perform star transformation for data warehouse tables based on the star schema. Transform BETWEEN to greater than or equal to and less than or equal to statements.
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CHAPTER 19 IMPROV IN G DA TA BAS E PERFORMAN CE: SQL QUERY OPTIMIZA TION
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Oracle can often access the same data through different paths. For each query, the optimizer evaluates all the available paths and picks the least expensive one in terms of resource usage. The following sections present a summary of the common access methods available to the optimizer. If joins are involved, then the join order and the join method are evaluated to finally arrive at the best execution plan. You ll take a brief look at the steps the optimizer goes through before deciding on its choice of execution path.
Full Table Scans
Oracle scans the entire table during a full table scan. Oracle reads each block in the table sequentially, so the full table scan can be efficient if the database uses a high default value internally for the DB_FILE_MULTIBLOCK_READ_COUNT initialization parameter. The parameter determines the maximum number of blocks the database reads during a sequential scan. However, for large tables, full table scans are inefficient in general.
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