Question

For example I have a table which is used for logging. So very old data is useless and there are no reasons to leave it in the table. I want create a trigger which will delete old rows if number of existing rows more than 10 for example. What I already have:

CREATE TABLE log (
    logId INT UNSIGNED NOT NULL AUTO_INCREMENT,
    firstLogin DATETIME NOT NULL,
    lastLogin DATETIME NOT NULL,
    fingerprint VARCHAR(64) CHARACTER SET BINARY,
    ip VARCHAR(24) NOT NULL,
    accountId INT UNSIGNED NOT NULL,
    FOREIGN KEY (accountId)
        REFERENCES accounts (accountId)
        ON UPDATE CASCADE ON DELETE CASCADE,
    PRIMARY KEY (logId)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

DELIMITER |

CREATE TRIGGER logbeforeinsert BEFORE INSERT ON log
    FOR EACH ROW 
        BEGIN
            SET @rowcount = (SELECT COUNT(*) FROM log WHERE accountId = NEW.accountId);
            IF @rowcount > 9 THEN
                DELETE FROM log WHERE accountId = NEW.accountId LIMIT 1;
            END IF;
        END;
|

DELIMITER ; 

But with this trigger inserting stopped at all after number of rows had reached 10.

Answer 1

Your trigger tries to write to the same table (DELETE is write access), that it is inserting into – this is not supported.

As you have a BEFORE INSERT trigger, failure of the trigger means failure of the INSERT.

You need either to trigger the delete from an operation, that does not write to log, or rethink your model.