Increment Table ID Field with Bitwise Counting

问题

I am working on a project where I need my ID column to be a power of 2 (1,2,4,8,16..). I know that we cannot offset the auto_increment but for simple addition/subtraction in my.cnf.

Example:

id
----
1
2
4
8
16
32
64
128
etc

One of the ideas I had was to use the auto increment functionality as the base, and then create a trigger to apply the power of 2 and update the new ID, but unfortunately, it is not working:

DELIMITER $$
CREATE TRIGGER testbitcompatid BEFORE INSERT ON Table
    FOR EACH ROW 
    BEGIN
        SET NEW.id = pow(NEW.id, 2)
    END;
$$
DELIMITER ;

Because the BEFORE INSERT has not yet generated the AUTO_INCREMENT id, the AUTO_INCREMENT will always return 0, essentially causing no change on the columns.

I also tried AFTER INSERT:

DELIMITER $$
CREATE TRIGGER testbitcompatid AFTER INSERT ON Table
    FOR EACH ROW 
    BEGIN
        SET Table.id = pow(NEW.id, 2) WHERE id = NEW.id;
    END;
$$
DELIMITER ;

But this failed because you cannot change values of the table which the trigger is applied to during an AFTER INSERT.

Scratching my head, but I am sure someone else has a great way of accomplishing this.

回答1:

To work around all the issues above, I was able to construct the following which works great!

DELIMITER $$
CREATE TRIGGER testbitcompatid BEFORE INSERT ON Table
    FOR EACH ROW 
    BEGIN
        SET @LAST_ROW = (SELECT MAX(id) FROM Table);
        SET NEW.id = CASE WHEN @LAST_ROW IS NULL THEN 1 ELSE @LAST_ROW * 2 END;

    END;
$$
DELIMITER ;

Pretty much, we take the highest id, grab the log(2) of it which gives us the corresponding AUTO_INCREMENT id. We then add 1, and power that up to 2.

I hope this helps prevent some headache down the road for others.

来源：https://stackoverflow.com/questions/12412043/increment-table-id-field-with-bitwise-counting

标签

mysql

triggers

bit-manipulation