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After a statement is explained, the explain results are stored in the PMDB. This information includes the objects referenced by the statement and the operations performed on these objects. The top statements are automatically explained every day. You can control how many statements to explain using a setting for the Explain Statements PMDB process in AdminPoint. For more information, see the Administration Guide.
Statements are also parsed by a proprietary Precise for Oracle parser. Parsing allows Precise for Oracle for example to highlight statement objects from the execution plan and to format execution plans. The number of statements that are parsed is again governed by a setting in the PMDB. For more information, see the Precise Administration Guide.
Understanding the execution plan chosen by the Oracle Optimizer is extremely important when tuning your application. You can ensure optimal system performance by ensuring that the best plans are used for your queries.
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If you are running Oracle 10g and later, you can also access the execution plan associated with a CREATE TABLE as SELECT statements. In addition to examining the plan, this lets you gain additional insight into a problem by analyzing the findings displayed for these statements.
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The statement will only explained. It will not be parsed. |
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To analyze an execution plan, Precise for Oracle provides you with a full picture of the objects (table, indexes, and so on) participating in the plan. You can change a plan by modifying your query or changing the schema, for example by adding an index. To assist in this, Precise for Oracle can provide you with index recommendations for your statement. |
One technique to identify the source of the problem is to view the historical information of statements, showing performance degradation. A change in the schema, volume, or execution plan may explain the impact on performance.
An important concept within the SQL tab is that of alternative or related SQL. You can take a statement and create different alternatives for it as part of the tuning process. There are a number of different ways of doing this. For example, you may re-write a statement yourself so that it gives the same result in a more efficient manner; or you may click Actions>Generate Alternatives to get Precise for Oracle to generate alternatives for you; or you may change a statement's execution plan by simulating the addition of an index in the What-If tab.
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Table 8-1 Launching the SQL tab in context
Tab Entities
Dashboard Statement
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| Tab | Entities |
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| Dashboard | Statement |
| Current | Statement, Active or Current Session that is currently executing a statement. |
| Activity |
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| Statement, PL/SQL |
| Objects |
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| Statement, PL/SQL |
| What- |
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| If | Statement that is affected by one or more index evaluations. |
In the Current, Activity, and Objects tabs, you can either launch the SQL tab by clicking the SQL tab button when the selected entity in the Main area is a statement, or by selecting a statement in the Association area and clicking the Tune icon that appears before the SQL text.
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For a PL/SQL, the SQL tab offers only limited functionality. |
If the SQL tab is opened with no statement in context, a message prompts you to open a statement.
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There can be statements with many execution plans. Only the latest 7 plans appear on the tree. Click More..., if available, to view additional execution plans. See “About the Dashboard tab” on page 43, “About the Current tab” on page 51, “About "About the Activity tab” on page 61, and “About the Objects tab” on page 87.*
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