1/26/2024 0 Comments Mysql optimizer hints![]() The online application upgrade hints suggest how to handle conflicting INSERT and UPDATE operations when performing an online application upgrade using edition-based redefinition: SELECT /*+ FIRST_ROWS(10) */ * FROM employees ฤก9.1.2.6 Hints for Online Application Upgrade For example, the following hint directs the optimizer to pick the query plan that produces the first 10 rows from the employees table at the lowest cost: ![]() A statement block can have only one comment containing hints, and that comment must follow the SELECT, UPDATE, INSERT, MERGE, or DELETE keyword. Oracle Database supports more than 60 hints, each of which may have zero or more parameters. For this reason, test by means of hints, but use other techniques to manage the SQL execution plans, such as SQL Tuning advisor and SQL Plan Baselines. Changes in the database and host environment can make hints obsolete or even have negative consequences. ![]() The disadvantage of hints is the extra code that must be managed, checked, and controlled. In this case, you may use hints to instruct the optimizer to use a better execution plan. For example, you may know that a certain index is more selective for certain queries. In a test or development environments, hints are useful for testing the performance of a specific access path. Hints enable you to make decisions normally made by the optimizer, sometimes causing the optimizer to select a plan that it sees as higher cost. When writing SQL, you may know information about the data unknown to the optimizer. A hint is an instruction to the optimizer.
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