数据库不一致状态MySQL / InnoDB

Question

I am wondering if it is necessary to use locking in most likely concurrent environment and how in following case . Using MySQL database server with InnoDB engine

Let's say I have a table

CREATE TABLE `A` (
    `id`    INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    `m_id`  INT NOT NULL, -- manual id
    `name`  VARCHAR(10)
)  ENGINE=INNODB;

And the procedure

CREATE PROCEDURE `add_record`(IN _NAME VARCHAR(10))
BEGIN
    DECLARE _m_id INT;
    DECLARE EXIT HANDLER FOR SQLEXCEPTION ROLLBACK;
    START TRANSACTION;

    SELECT (`m_id` + 1) INTO _m_id FROM `A` WHERE `id` = (SELECT MAX(`id`) FROM `A`);

    INSERT INTO `A`(`m_id`, `name`) VALUES(_m_id, _NAME);

    COMMIT;
END$$

Like you see the fact is that I am increasing m_id manually and concurrent transactions are most likely happening. I can't make my mind if database might become in inconsistent state. Also using FOR UPDATE and LOCK IN SHARE MODE has no point in this situation as transaction deals with new records and has nothing to do with updates on a specific row. Further LOCK TABLES are not allowed in stored procedures and is quite insufficient.

So, my question is how to avoid inconsistent state in marked scenario if it is possible to happen actually. Any advice will be grateful

Answer 1

transaction deals with new records and has nothing to do with updates on a specific row

Such a new record is known as a phantom :

phantom

A row that appears in the result set of a query, but not in the result set of an earlier query. For example, if a query is run twice within a transaction , and in the meantime, another transaction commits after inserting a new row or updating a row so that it matches the WHERE clause of the query.

This occurrence is known as a phantom read. It is harder to guard against than a non-repeatable read , because locking all the rows from the first query result set does not prevent the changes that cause the phantom to appear.

Among different isolation levels , phantom reads are prevented by the serializable read level, and allowed by the repeatable read , consistent read , and read uncommitted levels .

So to prevent phantoms from occurring on any statement , one can simply set the transaction isolation level to be SERIALIZABLE . InnoDB implements this using next-key locks , which not only locks the records that your queries match but also locks the gaps between those records .

The same can be accomplished on a per-statement basis by using locking reads , such as you describe in your question: LOCK IN SHARE MODE or FOR UPDATE (the former allows concurrent sessions to read the matching records while the lock is in place, whilst the latter does not).

Answer 2

First, a sequence table

  CREATE TABLE m_id_sequence (
      id integer primary key auto_increment
    );

and then alter the procedure to get the next m_id from the sequence table

DELIMITER $$
CREATE PROCEDURE `add_record`(IN _NAME VARCHAR(10))
BEGIN
    DECLARE _m_id INT;
    DECLARE EXIT HANDLER FOR SQLEXCEPTION ROLLBACK;
    START TRANSACTION;

    INSERT INTO m_id_sequence VALUES ();
    SET _m_id = LAST_INSERT_ID();

    INSERT INTO `A`(`m_id`, `name`) VALUES(_m_id, _NAME);

    COMMIT;
END$$
DELIMITER ;