Monday, June 11, 2018

Difference between SQL Profile and SPM(SQL Plan Management) Baselines

SQL Profile

In Oracle a SQL Profile creates extra information about a particular SQL that the optimizer can use at run time to select the optimal plan to ensure best performance. In essence the SQL Profile enables dynamic behavior where the optimizer has multiple plans to choose from at run time based on run time bind variables etc. When you run the SQL Tuning Advisor for the list of recommendation you will see that the recommendation specifies whether a SQL can be improved by creating a SQL Profile or SQL Baseline. It is preferable to choose a SQL Profile simply because it allows the optimizer to pick best execution plans at run time.

SQL Baseline

SQL Baseline on the other hand is more of a brute force method, when you simply marry a particular SQL to stay with a specific SQL execution plan. So no matter what the run time bind variables are for a given SQL, the optimizer will always try to use the SQL Baseline plan. This may work fine for most cases but instances where data skew is high it is preferable that one pick more efficient plans based on bind variable values passed at run time instead of always picking the same plan as generated by the SQL Baseline.

In any case knowledge of the application data may make you prefer one over the other, in case you are clueless about this, then I would suggest you stick with the SQL Profile.

How Does SQL Profiles Work?

When profiling a SQL statement, SQL Tuning Advisor uses a specific set of bind values as input.

The advisor compares the optimizer estimate with values obtained by executing fragments of the statement on a data sample. When significant variances are found, SQL Tuning Advisor bundles corrective actions together in a SQL profile, and then recommends its acceptance.

The corrected statistics in a SQL profile can improve optimizer cardinality estimates, which in turn leads the optimizer to select better plans. SQL profiles provide the following benefits over other techniques for improving plans:

Unlike hints and stored outlines, SQL profiles do not tie the optimizer to a specific plan or subplan. SQL profiles fix incorrect estimates while giving the optimizer the flexibility to pick the best plan in different situations.

Unlike hints, no changes to application source code are necessary when using SQL profiles. The use of SQL profiles by the database is transparent to the user.

How Does SQL Plan Management Work?

When a SQL statement is hard parsed, the cost based optimizer produces several execution plans and selects the one with the lowest cost. If a SQL plan baseline is present, the optimizer compares the plan it just produced with the plans in the SQL plan baseline. If a matching plan is found that is flagged as accepted the plan is used. If the SQL plan baseline doesn't contain an accepted plan matching the one it just created, the optimizer evaluates the accepted plans in the SQL plan baseline and uses the one with the lowest cost. If the execution plan originally produced by the optimizer has a lower cost than those present in the SQL plan baseline, it is added to the baseline as a not-accepted plan, so it is not used until it is verified not to cause a reduction in performance. If a system change affects all existing accepted plans, so they are considered non-reproducible, the optimizer will use the plan with the lowest cost.

Oracle call this a "conservative plan selection strategy", as the optimizer preferentially uses a tried an tested execution plan, even if a new plan looks like it might perform better. Only when the newer plan is proved to perform well will it be accepted for use.

The use of SQL plan baselines is controlled by the OPTIMIZER_USE_SQL_PLAN_BASELINES parameter, which is set to TRUE by default. Access to the DBMS_SPM package requires the ADMINISTER SQL MANAGEMENT OBJECT privilege.

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