The hints override any execution plan the query optimizer might select for a query. The SQL_NO_CACHE hint turns off MySQL’s built-in query caching mechanism for a particular query. References to const tables in join-order hints are ignored, although the hint … EXPLAIN shows a decision that was made based on the metadata that was available. Adding a hint allows you to force some other execution plans and compare the actual execution time. Hints limit the choices the optimizer has available, so if data in the tables change or new indexes are created, MySQL may be unable to adapt because of your hints. Optimizer hints are instructions that you can embed in your SQL that instruct or suggest to the optimizer how you would like the SQL to be executed. Earlier, changing a system variable for a single query would require four steps: 1) Store current value. In some cases, it might not have enough information about the data and plan queries in a non-optimal way. SET optimizer_switch = 'mrr_cost_based=off'; 3) Run the query. Using query hints is useful for two distinct reasons: In debugging. MySQL uses a cost-based optimizer to determine the best way to execute a query. There are different reasons for that. This hint allows to change the value of a system variable for a single query. An optimizer hint is a code snippet within an SQL statement controlling the decisions of the optimizer. In MySQL 8.0.3, we have introduced a new optimizer hint called SET_VAR. You can help MySQL make the query cache more efficent by using this hint on queries that are highly dynamic (such as a keyword search, or a report that only runs nightly).
Ask Question Asked 10 years, 9 months ago. SET @old_optimizer_switch = @@optimizer_switch; 2) Set new variable value. They should be used sparingly and with caution! Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. To ensure that optimizer hints are not stripped if you are using an older version of the mysql client with a version of the server that understands optimizer hints, invoke mysql with the --comments option.
The mysql client by default strips comments from SQL statements sent to the server (including optimizer hints) until MySQL 5.7.7, when it was changed to pass optimizer hints to the server. Let’s take a look at some MySQL Optimization Hints: SQL_NO_CACHE. The MySQL optimizer places const tables first in the join order, and the position of a const table cannot be affected by hints. It usually does a great job, but not all the time.
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... How to insert an “Optimizer hint” to Hibernate criteria api query. As well as adjusting the cost constants in the mysql system database, MySQL also provides a method to override how plans are selected. Hints give us the opportunity, in cases where we have superior knowledge about the database, to influence the optimizer. Caution Because the SQL Server query optimizer typically selects the best execution plan for a query, we recommend that