mysql – Distinct vs Group By

2021-09-29 21:30:45

我有两张这样的桌子.
‘order’表有21886行.

CREATE TABLE `order` (
  `id` bigint(20) unsigned NOT NULL,
  `reg_date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `idx_reg_date` (`reg_date`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci


CREATE TABLE `order_detail_products` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `order_id` bigint(20) unsigned NOT NULL,
  `order_detail_id` int(11) NOT NULL,
  `prod_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_order_detail_id` (`order_detail_id`,`prod_id`),
  KEY `idx_order_id` (`order_id`,`order_detail_id`,`prod_id`)
) ENGINE=InnoDB AUTO_INCREMENT=572375 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

我的问题在这里.

MariaDB [test]> explain
    -> SELECT DISTINCT A.id
    -> FROM order A
    -> JOIN order_detail_products B ON A.id = B.order_id
    -> ORDER BY A.reg_date DESC LIMIT 100, 30;
+------+-------------+-------+-------+---------------+--------------+---------+-------------------+-------+----------------------------------------------+
| id   | select_type | table | type  | possible_keys | key          | key_len | ref               | rows  | Extra                                        |
+------+-------------+-------+-------+---------------+--------------+---------+-------------------+-------+----------------------------------------------+
|    1 | SIMPLE      | A     | index | PRIMARY       | idx_reg_date | 8       | NULL              | 22151 | Using index; Using temporary; Using filesort |
|    1 | SIMPLE      | B     | ref   | idx_order_id  | idx_order_id | 8       | bom_20140804.A.id |     2 | Using index; Distinct                        |
+------+-------------+-------+-------+---------------+--------------+---------+-------------------+-------+----------------------------------------------+
2 rows in set (0.00 sec)

MariaDB [test]> explain
    -> SELECT A.id
    -> FROM order A
    -> JOIN order_detail_products B ON A.id = B.order_id
    -> GROUP BY A.id
    -> ORDER BY A.reg_date DESC LIMIT 100, 30;
+------+-------------+-------+-------+---------------+--------------+---------+-------------------+------+------------------------------+
| id   | select_type | table | type  | possible_keys | key          | key_len | ref               | rows | Extra                        |
+------+-------------+-------+-------+---------------+--------------+---------+-------------------+------+------------------------------+
|    1 | SIMPLE      | A     | index | PRIMARY       | idx_reg_date | 8       | NULL              |   65 | Using index; Using temporary |
|    1 | SIMPLE      | B     | ref   | idx_order_id  | idx_order_id | 8       | bom_20140804.A.id |    2 | Using index                  |
+------+-------------+-------+-------+---------------+--------------+---------+-------------------+------+------------------------------+

上面列出的是,两个查询返回相同的结果但不同的结果太慢(解释太多行).
有什么不同？

解决方法:

通常建议使用DISTINCT而不是GROUP BY,因为这是您真正想要的,并让优化器选择“最佳”执行计划.但是 – 没有优化器是完美的.使用DISTINCT,优化器可以为执行计划提供更多选项.但这也意味着它有更多选择来选择糟糕的计划.

你写的DISTINCT查询是“慢”,但你不告诉任何数字.在我的测试中(在MariaDB 10.0.19和10.3.13上有10倍的行),DISTINCT查询就像(仅)慢25％(562ms / 453ms). EXPLAIN结果完全没有帮助.它甚至“撒谎”.使用LIMIT 100,30,它需要读取至少130行(这是我的EXPLAIN实际上为GROUP BY所做的),但它显示了65.

我无法解释执行时间的25％差异,但似乎引擎在任何情况下都在进行全表/索引扫描,并在结果可以跳过100并选择30行之前对结果进行排序.

最好的计划可能是：

>按降序顺序逐个读取idx_reg_date索引(表A)中的行
>查看idx_order_id索引中是否存在匹配(表B)
>跳过100个匹配的行
>发送30个匹配的行
>退出

如果A中有10％的行在B中没有匹配,那么该计划将从A读取143行.

我能以某种方式强制推行这项计划的最佳方法是：

SELECT A.id
FROM `order` A
WHERE EXISTS (SELECT * FROM order_detail_products B WHERE A.id = B.order_id)
ORDER BY A.reg_date DESC
LIMIT 30
OFFSET 100

此查询在156毫秒内返回相同的结果(比GROUP BY快3倍).但那仍然太慢了.它仍然可以读取表A中的所有行.

我们可以证明一个更好的计划可以存在一个“小”子查询技巧：

SELECT A.id
FROM (
    SELECT id, reg_date
    FROM `order`
    ORDER BY reg_date DESC
    LIMIT 1000
) A
WHERE EXISTS (SELECT * FROM order_detail_products B WHERE A.id = B.order_id)
ORDER BY A.reg_date DESC
LIMIT 30
OFFSET 100

此查询在“no time”(~0 ms)内执行,并在我的测试数据上返回相同的结果.尽管它不是100％可靠,但它表明优化器并没有做得很好.

那么我的结论是什么：

>优化器并不总是做得最好,有时需要帮助
>即使我们知道“最佳计划”,我们也不能总是强制执行
> DISTINCT并不总是比GROUP BY快
>当没有索引可以用于所有子句时 – 事情变得非常棘手

测试模式和虚拟数据：

drop table if exists `order`;
CREATE TABLE `order` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `reg_date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `idx_reg_date` (`reg_date`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

insert into `order`(reg_date)
    select from_unixtime(floor(rand(1) * 1000000000)) as reg_date
    from information_schema.COLUMNS a
       , information_schema.COLUMNS b
    limit 218860;

drop table if exists `order_detail_products`;
CREATE TABLE `order_detail_products` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `order_id` bigint(20) unsigned NOT NULL,
  `order_detail_id` int(11) NOT NULL,
  `prod_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_order_detail_id` (`order_detail_id`,`prod_id`),
  KEY `idx_order_id` (`order_id`,`order_detail_id`,`prod_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

insert into order_detail_products(id, order_id, order_detail_id, prod_id)
    select null as id
    , floor(rand(2)*218860)+1 as order_id
    , 0 as order_detail_id
    , 0 as prod_id
    from information_schema.COLUMNS a
       , information_schema.COLUMNS b
    limit 437320;

查询：

SELECT DISTINCT A.id
FROM `order` A
JOIN order_detail_products B ON A.id = B.order_id
ORDER BY A.reg_date DESC
LIMIT 30 OFFSET 100;
-- 562 ms

SELECT A.id
FROM `order` A
JOIN order_detail_products B ON A.id = B.order_id
GROUP BY A.id
ORDER BY A.reg_date DESC
LIMIT 30 OFFSET 100;
-- 453 ms

SELECT A.id
FROM `order` A
WHERE EXISTS (SELECT * FROM order_detail_products B WHERE A.id = B.order_id)
ORDER BY A.reg_date DESC
LIMIT 30 OFFSET 100;
-- 156 ms

SELECT A.id
FROM (
    SELECT id, reg_date
    FROM `order`
    ORDER BY reg_date DESC
    LIMIT 1000
) A
WHERE EXISTS (SELECT * FROM order_detail_products B WHERE A.id = B.order_id)
ORDER BY A.reg_date DESC
LIMIT 30 OFFSET 100;
-- ~ 0 ms

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