How to Enforce a Hard Row Limit in MySQL Tables?

Implementing Hard Row Limit in MySQL Tables

Question: Designers often face the need to enforce a maximum limit on rows stored in a MySQL table. Is there a definitive method to achieve this restriction beyond merely specifying a hint?

Answer: Setting a maximum row limit is a common database challenge. While the MAX_ROWS property acts as a suggestion rather than an absolute cap, there are indeed ways to impose a hard limit on MySQL tables:

Trigger-Based Mechanism:

• Create a BEFORE INSERT trigger.
• Count the number of existing rows in the table.
• If the count exceeds the set limit (e.g., 25), raise an error to prevent the insertion.

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 ;

Note: COUNT operations can slow down on large InnoDb tables.

MySQL 5.5 Signal/Resignal Statements:

• Raise an error using SIGNAL or RESIGNAL statements. This option is available in MySQL 5.5 onward.

Cron Script Method:

• Clear the table periodically using a cron script. This is a simpler but less automated solution.

Additional Considerations:

• Consider the performance impact of the COUNT operation in the trigger-based approach.
• Ensure proper error handling and logging in all methods to avoid data loss or inconsistencies.

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