MySQL FLOOR 结果不递增

Question

我有一个手动递增 ID 的过程，方法是向 ID 值迭代添加一个随机数。有时这会导致 ID 不递增，即使我可以确认语句 (FLOOR(RAND()*(1000-2+1))+2) 总是 returns 2-1000 之间的值。然而下面的简单语句：

SET `nextid` = `nextid` + (FLOOR(RAND()*(1000-2+1))+2);

仍然时不时增加 nextid 失败。如果我将我的 (FLOOR(RAND()*(1000-2+1))+2) 值分成一个单独的变量并添加这两个变量，这不会发生。下面是重现该行为的最小示例测试用例。

DELIMITER $$
CREATE PROCEDURE `test`()
BEGIN
    DECLARE `nextid` BIGINT DEFAULT 793991813529600000;
    DECLARE `previous` BIGINT DEFAULT 0;
    DECLARE `i` BIGINT DEFAULT 0;
    DECLARE `random` BIGINT DEFAULT 0;

    WHILE `i` < 100000 DO
        SET `previous` = `nextid`;
        -- Produce a random number between 2 and 1000.
        SET `random` = (FLOOR(RAND()*(1000-2+1))+2);
        SET `nextid` = `nextid` + `random`;

        -- Error if the nextid is no different from the previous ID.
        -- This is successful every time. 
        IF `nextid` = `previous` THEN
            SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Oh no!', MYSQL_ERRNO = 2000;
        END IF;

        SET `i` = `i` + 1;
    END WHILE;

    SET `i` = 0;
    WHILE `i` < 100000 DO
        SET `previous` = `nextid`;
        -- Increment the ID by a random number and do the set in a single statment.
        SET `nextid` = `nextid` + (FLOOR(RAND()*(1000-2+1))+2);

        -- This fails randomly, but reliably.
        IF `nextid` = `previous` THEN
            SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Whoops!', MYSQL_ERRNO = 2000;
        END IF;

        SET `i` = `i` + 1;
    END WHILE;

END $$
DELIMITER ;

CALL `test`();
DROP PROCEDURE IF EXISTS `test`;

看起来可能是某种类型的转换问题，但我找不到任何可以确认的内容。所以我的问题是，是什么导致了这种行为？

其他详细信息；

mysql --version
mysql  Ver 14.14 Distrib 5.7.24, for Linux (x86_64) using  EditLine wrapper

Answer 1

以下是最有可能发生的情况：

RAND() returns 一个浮点数
FLOOR(FLOAT VALUE) returns 一个浮点数
BIGINT VALUE + FLOAT VALUE returns 一个浮点数
浮点数不精确，这就是你遇到问题的地方

对我来说失败的 nextid 值之一是 793991813579709184。看看当你向它添加 50f 时会发生什么......浮点值不增加：

CREATE TABLE t (i BIGINT);
INSERT INTO t VALUES (793991813579709184);
INSERT INTO t VALUES (793991813579709184 + 50e0);
SELECT * FROM t;

| 793991813579709184 |
| 793991813579709184 |

在您的测试代码中，我更改了这一行并且它按预期工作：

SET `nextid` = `nextid` + CAST(RAND() * 999 AS SIGNED) + 2;

MySQL FLOOR 结果不递增

MySQL Arithmetic with FLOOR result does not increment

mysql

floating-point

stored-procedures

function

integer-arithmetic