MySQL索引的问题

Question

可为什么MySQL是不是在下面的查询

SELECT `Slugs`.`slug` FROM `slugs` AS `Slugs` 
WHERE `Slugs`.`country_id` = 1 AND `Slugs`.`expired` = 0  
LIMIT 308400,300 

我已经创造了在where子句

表结构所指向的两列的cmposite指数使用正确的索引有人告诉我为表slugs

CREATE TABLE IF NOT EXISTS `slugs` (
    `id` int(11) NOT NULL AUTO_INCREMENT, 
    `slug` varchar(255) NOT NULL, 
    `post_fields` text NOT NULL, 
    `slugdata` text NOT NULL, 
    `updated_date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, 
    `country_id` int(11) NOT NULL DEFAULT '1', 
    `expired` int(1) NOT NULL DEFAULT '0', 
    PRIMARY KEY (`id`), 
    UNIQUE KEY `slug_2` (`slug`,`country_id`), 
    KEY `updated_date` (`updated_date`), 
    KEY `country_id` (`country_id`), 
    KEY `slug` (`slug`), 
    KEY `expired` (`expired`), 
    KEY `country_id_2` (`country_id`,`expired`) 
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1612485 ; 

指标：

Keyname  Type Unique Packed Field  Cardinality  Collation Null Comment 
PRIMARY  BTREE Yes  No  id    1406994  A  
slug_2  BTREE Yes  No  slug    1406994  A  
            country_id  1406994  A 
updated_date BTREE No  No  updated_date   21  A  
country_id BTREE No  No  country_id   21  A  
slug   BTREE No  No  slug    1406994  A  
expired  BTREE No  No  expired    21  A  
country_id_2 BTREE No  No  country_id   21  A  
            expired    21  A 

输出解释

id select_type  table type possible_keys key  key_len  ref  rows Extra 
1 SIMPLE Slugs ref  country_id,expired,country_id_2  country_id 4 const 670284 Using where 

Answer 1

你是什么意思它不使用正确的索引？

country_id和country_id_2的基数非常低，实际上两个索引都相同 - 因此使用基于2列的索引没有任何好处。这意味着很少有记录过期<> 0。

但是为什么过期指数的基数也是21？

什么是从输出：

SELECT 'expired' AS fld 
, COUNT(*) AS distinct_values 
, AVG(n) AS rows 
FROM 
(SELECT expired AS v 
, COUNT(*) AS n 
FROM slugs 
GROUP BY expired) ilv1 
UNION 
SELECT 'country_id', 
, COUNT(*) AS distinct_values 
, AVG(n) AS rows 
FROM 
(SELECT country_id AS v 
, COUNT(*) AS n 
FROM slugs 
GROUP BY country_id) ilv2 
    SELECT 'expired:country_id', 
, COUNT(*) AS distinct_values 
, AVG(n) AS rows 
FROM 
(SELECT CONCAT(expired, country_id) AS v 
, COUNT(*) AS n 
FROM slugs 
GROUP BY CONCAT(expired, country_id)) ilv3;