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SQL Plan Baselines
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Under SQL Plan Management, the database maintains a plan history, which is a record of all SQL plans generated over time for a SQL statement by the optimizer. The optimizer uses the plan history to figure out the optimal execution plan for a statement. Not all plans in the plan history for a statement
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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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are acceptable plans, however. The database defines as acceptable only those execution plans that don t lead to deterioration in performance relative to other plans in the plan history. The SQL plan baseline for a SQL statement is the set of all accepted plans in the plan history for a statement. The very first plan for a statement is always an accepted plan, because there s nothing to compare it with. So, the SQL plan baseline and the plan history for a new SQL statement are identical at this point. Newer execution plans for the statement will automatically be a part of the statement s plan history, but are added to the SQL plan baseline for the statement only if the new plans are verified not to cause a regression in performance. The Automatic SQL Tuning Adviser task, which is a part of the automate maintenance tasks, verifies SQL plans. The advisor looks for high-load SQL statements and stores the accepted plans for those statements in that s statement s SQL plan baseline. You can manage SQL plan baselines by using the DBMS_SPM package or through Enterprise Manager. I explain the steps in the following sections.
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There are two ways to capture the SQL plan baselines: have the database automatically capture the plans or load them in the database yourself. I explain both techniques in this section.
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By default, the database doesn t maintain a plan history for SQL statements you execute. You must set the initialization parameter OPTIMIZER_CAPTURE_SQL_PLAN_BASELINES to TRUE (the default value is FALSE) for the database to start capturing the SQL plan baselines. When you set the parameter to TRUE, the database automatically creates and maintains a plan history for all repeatable SQL statements that are executed in the database.
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Tip By using the SQL Performance Analyzer (see 20), you can find out which SQL statements are likely to regress following a database upgrade to, say, Oracle Database 11g Release 1 from Oracle Database 10g Release 2. You can capture the execution plans for these statements and load them into the SQL management base of the upgraded database, thus avoiding the performance regression.
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Manual Plan Loading
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You can also load SQL plans manually into the SQL plan baselines. When you load plans manually, the database loads them automatically as accepted plans, without verifying the performance of the plans. You can bulk load SQL plans you captured before upgrading the database into a SQL plan baseline after upgrading your database to a new release. You can use either a SQL Tuning Set (STS) or load the plans from the database cursor cache. I show both techniques in the following sections. Execute the DBMS_SPM function LOAD_PLANS_FROM_SQLSET in order to load SQL plans from an STS. First create an empty STS as shown here: begin dbms_sqltune.create_sqlset( sqlset_name => 'testset1', description => 'Test STS to capture AWR Data'); end; / Next, load the new STS with SQL statements from the Automatic Workload Repository (AWR) snapshots.
CHAPTER 19 IMPROV IN G DA TA BAS E PERFORMAN CE: SQL QUERY OPTIMIZA TION
declare baseline_cur dbms_sqltune.sqlset_cursor; begin open baseline_cur for select value(p) from table (dbms_sqltune.select_workload_repository( 'peak baseline',null,null,'elapsed_time',null,null,null,20)) p; dbms_sqltune.load_sqlset ( sqlset_name => 'testset1', populate_cursor => baseline_cur); end; / The STS shown in this example includes the top 20 statements from the AWR peak baseline, selected based in the criterion of elapsed time. The ref cursor and the table function help select the top 20 statements from the AWR baseline. Load the SQL plans from the STS into the SQL plan baseline by executing the LOAD_PLANS_FROM_SQLSET function. declare test_plans pls_integer; begin test_plans := dbms_spm.load_plans_from_sqlset( sqlset_name => 'testset1'); end; / You can also use the cursor cache instead of an STS as the source of the SQL plans you want to load into a SQL plan baseline. The following example shows how to load SQL plans from the cursor cache using the LOAD_PLANS_FROM_CURSOR_CACHE function. declare test_plans pls_integer; begin test_plans := dbms_spm.load_plans_from_cursor_cache ( sql_id => '123456789999') return pls_integer; end; /
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