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Optimizer Hints

Oracle Tips by Burleson Consulting

Optimizer mode hints are used to redirect the overall optimizer goal. The most common mode for SQL tuning is the rule hint, which is used when you suspect that the CBO is using a suboptimal execution plan. It should be noted that the optimizer hints only cause Oracle to apply the optimizer to the SQL, and additional hints can still be added for further tuning.

The all_rows Hint

The all_rows hint represents the cost-based approach designed to provide the best overall throughput and minimum resource consumption. The all_rows mode favors full-table scans and is not suitable for OLTP databases. When adding this hint to a rule-based database, make sure that all tables and indexes that participate in the query have statistics with the analyze command.

The rule Hint

The rule hint directs Oracle to apply rule-based optimization to the query. This hint is generally the first one to try when tuning a SQL statement where you suspect that the CBO is generating a suboptimal execution plan.

The rule hint ignores the table and index statistics and uses basic heuristics to generate an execution plan. Up until the release of Oracle8i, the rule-based optimizer often generated a faster execution plan than cost-based optimization.

The first_rows Hint

This is the cost-based approach designed to provide the best response time. Just as with the guidelines for adding the all_rows hint, make sure that all tables and indexes that participate in the query have statistics with the analyze command. The first_rows hint is useful when testing the execution plan for a database with a questionable optimizer mode such as choose. By using the first_rows hint, you can tune your SQL for what the CBO considers to be the fastest response time. Generally, all SQL running under optimizer_mode=choose should be tested with the first_rows and rule hints to see if the performance will improve.

Next, let’s look at how hints can be used improved to the efficiency of a table join.

This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.

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