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不久前，我开始优化 mysql db 的查询。我创建了一些索引并决定使用语法中的show index查看它们的参数。并看到基数不等于某些列的不同计数。

所以，我有桌子 

CREATE TABLE `item_owners` (
  `uid` varchar(255) NOT NULL,
  `version` bigint(20) NOT NULL,
  `type_id` varchar(255) NOT NULL,
  `owner_name` varchar(255) NOT NULL,
  `item_id` varchar(255) NOT NULL,
  `status_id` varchar(255) NOT NULL,
  PRIMARY KEY (`uid`),
  KEY `FK181EADBC7346EE24` (`status_id`),
  KEY `FK181EADBC90094D43` (`type_id`),
  KEY `FK181EADBC499E38CA` (`item_id`),
  CONSTRAINT `FK181EADBC499E38CA` FOREIGN KEY (`item_id`) REFERENCES `items` (`uid`),
  CONSTRAINT `FK181EADBC7346EE24` FOREIGN KEY (`status_id`) REFERENCES `statuses` (`uid`),
  CONSTRAINT `FK181EADBC90094D43` FOREIGN KEY (`type_id`) REFERENCES `types` (`uid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

在那个表中，我有 2M 行（实际上是 2M uniq PK）、500k uniq item_id、5 uniq status_id 和 20 uniq type_id

但是当我执行

show index from item_owners

我有

+-------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table       | Non_unique | Key_name           | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| item_owners |          0 | PRIMARY            |            1 | uid         | A         |     1893427 |     NULL | NULL   |      | BTREE      |         |               |
| item_owners |          1 | FK181EADBC7346EE24 |            1 | status_id   | A         |          17 |     NULL | NULL   |      | BTREE      |         |               |
| item_owners |          1 | FK181EADBC90094D43 |            1 | type_id     | A         |          17 |     NULL | NULL   |      | BTREE      |         |               |
| item_owners |          1 | FK181EADBC499E38CA |            1 | item_id     | A         |      148000 |     NULL | NULL   |      | BTREE      |         |               |
+-------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

然后我决定执行

analyze table item_owners

在这个输出之后是：

+-------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table       | Non_unique | Key_name           | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| item_owners |          0 | PRIMARY            |            1 | uid         | A         |     2005419 |     NULL | NULL   |      | BTREE      |         |               |
| item_owners |          1 | FK181EADBC7346EE24 |            1 | status_id   | A         |          17 |     NULL | NULL   |      | BTREE      |         |               |
| item_owners |          1 | FK181EADBC90094D43 |            1 | type_id     | A         |          17 |     NULL | NULL   |      | BTREE      |         |               |
| item_owners |          1 | FK181EADBC499E38CA |            1 | item_id     | A         |     2005419 |     NULL | NULL   |      | BTREE      |         |               |
+-------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

有谁知道这怎么可能？，因为我真的误解了为什么基数不等于 uniq 行数，即使是 PK 索引。

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1 回答 1

4

引用手册：

基数

对索引中唯一值数量的估计。这通过运行ANALYZE TABLE或更新myisamchk -a。基数是根据存储为整数的统计信息计算的，因此即使对于小型表，该值也不一定准确。

于 2013-05-24T12:28:19.410 回答