admin

糟糕的MySQL LEFT JOIN性能,逐组最大化

sql

我有下表

CREATE TABLE `prod_prices` (
  `id` varchar(32) COLLATE utf8_unicode_ci NOT NULL,
  `date` date NOT NULL,
  `price` decimal(10,2) NOT NULL,
  PRIMARY KEY (`id`,`date`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

还有一些样本数据

INSERT INTO `prod_prices` (`id`, `date`, `price`) VALUES
('plan_a', '2012-06-15', 10.20),
('plan_a', '2012-06-16', 10.30),
('plan_b', '2012-06-15', 5.20),
('plan_b', '2012-06-16', 5.30),
('plan_b', '2012-06-17', 5.50);

并想知道每个计划的最新价格。我有以下查询

SELECT p1.id, p1.date, p1.price
FROM prod_prices p1
LEFT JOIN prod_prices p2 ON p1.id = p2.id
AND p1.date < p2.date
WHERE p2.id IS NULL

尽管可以产生真正的糟糕表现,但这种方法还是有效的。 EXPLAIN表演

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE          p1      ALL     NULL             NULL    NULL        NULL   5   
1   SIMPLE          p2      ref     PRIMARY         PRIMARY     98       p1.id   1      Using where; Using index; Not exists

因此p1表选择不使用任何索引。什么事?


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2021-07-01

共1个答案

admin

这应该给出相同的结果,并且性能要好得多。

SELECT  p1.*
FROM    prod_prices p1
        INNER JOIN
        (   SELECT  ID, MAX(Date) AS Date
            FROM    prod_prices
            GROUP BY ID
        ) AS p2
            ON p1.ID = p2.ID 
            AND p1.Date = p2.Date
2021-07-01