How Can You Effectively Limit Row Count in MySQL Tables Beyond the MAX_ROWS Hint?

Limiting Row Count in MySQL Tables: Beyond the MAX_ROWS Hint

While MySQL allows specifying a MAX_ROWS parameter when creating tables, it serves as a mere suggestion for the database engine. Tables can exceed this limit, rendering it ineffective as an absolute cap on row count.

For true enforcement of a maximum row count, alternative approaches are necessary. One such method involves creating a BEFORE INSERT trigger that checks the current row count in the table:

DELIMITER $$

CREATE TRIGGER trigger1
BEFORE INSERT
ON table1
FOR EACH ROW
BEGIN
  SELECT COUNT(*) INTO @cnt FROM table1;
  IF @cnt >= 25 THEN
    CALL sth(); -- raise an error
  END IF;
END
$$

DELIMITER ;

This trigger evaluates the table's row count before inserting a new row. If the count meets or exceeds a predefined limit (e.g., 25), it triggers an error, effectively preventing the insertion.

However, this approach can introduce performance penalties, especially for large tables, as the COUNT operation can be computationally expensive.

MySQL 5.5 introduces SIGNAL // RESIGNAL statements as another means of raising errors. This can be incorporated into the trigger as follows:

CREATE TRIGGER trigger1
BEFORE INSERT
ON table1
FOR EACH ROW
BEGIN
  IF 25 <= ROW_COUNT() THEN
    SIGNAL SQLSTATE '23000' SET MESSAGE_TEXT = 
    'Table size limit reached.'; // Raise an error
  END IF;
END

This technique offers improved performance by leveraging the optimized COUNT(*) function on InnoDb tables, enhancing the overall efficiency of the row count enforcement mechanism.

The above is the detailed content of How Can You Effectively Limit Row Count in MySQL Tables Beyond the MAX_ROWS Hint?. For more information, please follow other related articles on the PHP Chinese website!