The Query Rewriter Plugin in MySQL is a component that allows you to modify incoming SQL queries before execution. It provides the ability to transform, route, filter, or expand queries based on specific requirements. This plugin operates at the SQL layer and can be utilized for optimizing query performance, enforcing security policies, implementing data partitioning strategies, or adding additional business logic to queries. With the Query Rewriter Plugin, you have the power to customize and shape SQL queries to meet your specific needs, providing flexibility and control over query execution within the MySQL server.
The Query Transformation feature enables you to rewrite or transform the original query into an equivalent or more efficient form. This can be useful for optimizing performance, simplifying complex queries, or enforcing certain query plans.
You must install Query Rewriter Plugin before using this feature, the concept of Query Rewriter is simple, it is a set of predefined SQL statements that is used to replace a certain pattern of SQL statements that is fired from your application programs.
If you have installed the plugin, the following SQL statements can be used to defined your SQL replacement rules and error message handling.
INSERT INTO query_rewrite.rewrite_rules (message, pattern, replacement)
VALUES(Unique_ID, Original_SQL, Rewrite_SQL);
The query_rewrite.rewrite_rules table in MySQL stores the rules used by the Query Rewriter Plugin to rewrite SQL queries. The table has two columns:
Pattern – This column represents the pattern or condition that triggers the rewriting of a SQL query. It defines the specific query or query pattern to match.
Replacement – This column specifies the replacement or transformation that should be applied to the matched query or query pattern.
When a SQL query is executed, the Query Rewriter Plugin checks the query_rewrite.rewrite_rules table for matching patterns. If a pattern matches the executed query, the plugin rewrites the query using the corresponding replacement. This allows you to modify the query structure, optimize it, or add custom logic based on specific patterns or conditions.
I utilize the message column to define a temporary unique id for the SQL replacement rule, so the actual rule id can be extracted with the following SQL.
SELECT id into :SID FROM query_rewrite.rewrite_rules where message=Unique_ID;
When you make changes to the query rewrite rules in the query_rewrite.rewrite_rules table, those changes are not immediately applied. Instead, MySQL caches the rules in memory for better performance. However, if you want to ensure that the updated rules take effect immediately, you can call the query_rewrite.flush_rewrite_rules() function.
If a load error occurs, the plugin also sets the Rewriter_reload_error status variable to ON and the error message will be stored in the Message column.
SELECT message FROM query_rewrite.rewrite_rules where id=:SID;
Actually, the Query Rewriter Plugin is powerful and easy to use. The most challenging aspect is finding a replacement SQL for your poorly performing SQL statement. Tosska DB Ace Enterprise for MySQL can assist you in automating this process, from identifying poorly performing SQL statements to rewriting SQL syntax and deploying replacement rules.