Oracle SQL profile and baselines

The differences between SQL profile and baselines in Oracle® and explains how it works when tuning a query.

Optimizer, profile, and baseline

At a high level, these three elements work together as follows:

• The query optimizer uses information like system statistics, bind variables, and compilations to get the best plan for query execution. However, sometimes input defects lead to a sub-optimal plan.

• SQL profiles contain auxiliary information that mitigates this issue. They minimize these mistakes and help the optimizer select the best plan.

• SQL plan baselines for SQL statements consist of a set of accepted plans. After parsing the statement, the optimizer selects the best plan from the set of accepted plans. If the cost optimizer finds another good plan, it adds the new plan to the plan history. However, the optimizer doesn’t use the new plan until it verifies that it will perform better than the currently accepted plan.

Think of it this way: SQL profiles give information to the optimizer to help select the best plan but don’t force the optimizer to select any specific plan. SQL plan baselines limit the optimizer plan selection to a set of accepted plans. If you want to consider cost-based plans, include them in the accepted baseline set of plans.

Use SQL profiles if you want the optimizer to use a least-cost plan and need to use the latest statistics. 

Use baselines when you want to use one of a specific set of plans. 

If SQL plan baselines can’t get the best plan from the accepted set, use the SQL profile instead.

SQL Plan Management

SQL Plan Management (SPM) has the following components:

• Plan capture
• Plan selection
• Plan evolution

SPM PLAN CAPTURE

When you execute any statement, the system hard parses it and generates a cost plan according to the available SQL profile. After it selects a cost-based plan, it compares the plans available in the SQL plan baselines. If a cost-based generated plan matches one of the accepted plans, then you can use that plan. If the plan does not match, the system adds it to the plan baselines as an unaccepted plan.

SPM PLAN SELECTION

When you execute the SQL statement with a baseline plan, it opts for the best plan for that SQL. The optimizer uses the same process. The available SQL profile also affects the estimated cost for each plan and selects a plan accordingly.

SPM PLAN EVOLUTION

The last component of SPM is the evolution of unaccepted plans, which tests the unaccepted plans against the accepted plans. This process assesses the best plan considering time taken by a query and the CPU resources required. It accepts the best plan according to the cost of the query. If an SQL profile is available, then it affects the estimated cost.

Profile versus baseline comparison

The following table, from https://www.cnblogs.com/princessd8251/articles/3637461.html, compares SQL profiles and SQL plan baselines:

Source :- https://docs.rackspace.com/blog/oracle-sql-profile-and-baselines/

Conclusion

When you execute any SQL statement, the optimizer creates an execution plan to parse the query, retrieve the data from the hard disk, and place it in memory. The SQL profile and baselines guide the optimizer to choose the least costly plan in terms of time and CPU cost. A good SQL plan efficiently runs a query and provides the desired results faster.