MySQL 在插入后触发器错误中删除行

Question

我在 MySQL 数据库中有三个 table，bids、offers 和 matched。 matched table 包含已匹配的出价和报价。有一个事件运行逻辑来执行下面显示的匹配。

CREATE EVENT bid_offer_matching
ON SCHEDULE EVERY 1 SECOND  
STARTS CURRENT_TIMESTAMP
DO
    INSERT INTO Matched(
        consumer_id,        /* Bidder */
        producer_id,        /* Offer */
        bid_id,             /* bid id */
        offer_id,           /* offer id */ 
        volume,             /* Volume */
        price,              /* Price */
        market_time)        /* Market Time */
    SELECT  
        bids.user_id,               /* Bidder */
        offers.user_id,             /* Offer */
        bids.bid_id,                /* Bid ID */
        offers.offer_id,            /* Offer ID */
        bids.bid_volume,            /* Volume */
        bids.bid_price,             /* Price */
        bids.market_time            /* Market Time */
    FROM  
        Bids bids
    INNER JOIN (
        SELECT *, ROW_NUMBER() 
            OVER (
                PARTITION BY 
                    offer_volume /* Partition by the volume in the offer */
                ORDER BY 
                    /* Order each partiton by the price low to high, this makes row 1 in each partition the lowest price offer*/ 
                    offer_price 
            )rn /* Get the row number also back */ 
        FROM Offers
    ) offers
    ON
        /* Volume must be equal */ 
        bids.bid_volume = offers.offer_volume AND
        /*Price must be at least asking price */ 
        bids.bid_price >= offers.offer_volume AND
        /* Market time must be same */ 
        bids.market_time >= offers.market_time
    WHERE
        /* This is important, it matches only the above condition to the lowest offer price resulting in best price for bidder */
        offers.rn = 1;

当我在 matched table 上创建触发器以删除从 bids 和 offers table 中插入的出价和报价时出现问题.我想删除它们，因为它们一旦匹配就不再需要了。此触发器的逻辑如下所示。

/*Crete the trigger*/
DELIMITER $$
CREATE TRIGGER match_insert_trigger
AFTER INSERT
ON Matched FOR EACH ROW 
BEGIN 
    /* DELETE THE BID*/
    DELETE FROM 
        Bids
    WHERE 
        bid_id=NEW.bid_id; /* The bid ID is inserted into the matched table to be used as reference, not a foreign key though */

    /* DELETE THE OFFER */
    DELETE FROM 
        Offers
    WHERE 
        offer_id=NEW.offer_id; /* The offer ID is inserted into the matched table to be used as reference, not a foreign key though */
END$$

DELIMITER ;

但是，当尝试执行事件以匹配出价和报价时，现在会发生错误

Error Code: 1442. Can't update table 'Bids' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

我只是在 match_insert_trigger 事件中更新 Bids table，但出价 table 并未用于触发任何事情，所以有点不确定这里出了什么问题？

编辑

我通过放置删除事件 bid_offer_matching 中匹配的 bids/offers 的逻辑实现了我需要的功能。但是我仍然不明白上面的错误是怎么发生的，如果有人知道的话。

下面是新的活动代码

DELIMITER $$
CREATE EVENT bid_offer_matching
ON SCHEDULE EVERY 1 SECOND  
STARTS CURRENT_TIMESTAMP
DO
BEGIN
    INSERT INTO Matched(
        consumer_id,        /* Bidder */
        producer_id,        /* Offer */
        bid_id,             /* bid id */
        offer_id,           /* offer id */ 
        volume,             /* Volume */
        price,              /* Price */
        market_time)        /* Market Time */
    SELECT  
        bids.user_id,               /* Bidder */
        offers.user_id,             /* Offer */
        bids.bid_id,                /* Bid ID */
        offers.offer_id,            /* Offer ID */
        bids.bid_volume,            /* Volume */
        bids.bid_price,             /* Price */
        bids.market_time            /* Market Time */
    FROM  
        Bids bids
    INNER JOIN (
        SELECT *, ROW_NUMBER() 
            OVER (
                PARTITION BY 
                    offer_volume /* Partition by the volume in the offer */
                ORDER BY 
                    /* Order each partiton by the price low to high, this makes row 1 in each partition the lowest price offer*/ 
                    offer_price 
            )rn /* Get the row number also back */ 
        FROM Offers
    ) offers
    ON
        /* Volume must be equal */ 
        bids.bid_volume = offers.offer_volume AND
        /*Price must be at least asking price */ 
        bids.bid_price >= offers.offer_volume AND
        /* Market time must be same */ 
        bids.market_time >= offers.market_time
    WHERE
        /* This is important, it matches only the above condition to the lowest offer price resulting in best price for bidder */
        offers.rn = 1;

/* *****DELETE STATEMENT ***** */
    /* DELETE THE OFFERS */
    DELETE FROM  
        Offers 
    WHERE 
        offer_id  
    IN 
        (SELECT offer_id FROM Matched);
        
    /* DELETE THE BIDS */
    DELETE FROM  
        Bids 
    WHERE 
        bid_id  
    IN 
        (SELECT bid_id FROM Matched);

END$$
        
DELIMITER ;

Answer 1

您的事件主体中的代码执行 INSERT INTO Matched... SELECT ... FROM Bids...

https://dev.mysql.com/doc/refman/8.0/en/stored-program-restrictions.html 说：

A stored function or trigger cannot modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.

请注意它说 “用于阅读或写作”。 这意味着通过阅读来自 table 的投标 table在你的 INSERT...SELECT 语句中做的，这与你不能修改触发器中的 tables 的规则冲突，这些触发器正在调用触发器的语句中读取。

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