mysql – 疯狂基数计算显示表状态

Iteration1：

mysql> show table status LIKE "mybigusertable";
+-----------------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+--------------------------+
| Name            | Engine | Version | Row_format | Rows    | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time         | Update_time | Check_time | Collation       | Checksum | Create_options | Comment                  |
+-----------------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+--------------------------+
| mybigusertable | InnoDB |      10 | Compact    | 3089655 |           1686 |  5209325568 |               0 |    797671424 |         0 |        3154997 | 2011-12-04 03:46:43 | NULL        | NULL       | utf8_unicode_ci |     NULL |                | InnoDB free: 13775872 kB | 
+-----------------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+--------------------------+

mysql> show index from mybigusertable;
+-----------------+------------+-----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table           | Non_unique | Key_name        | Seq_in_index | Column_name        | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------------+------------+-----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+
| mybigusertable |          0 | PRIMARY         |            1 | someid         | A         |     3402091 |     NULL | NULL   |      | BTREE    

迭代2

mysql> show index from mybigusertable;
+-----------------+------------+-----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table           | Non_unique | Key_name        | Seq_in_index | Column_name        | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------------+------------+-----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+
| mybigusertable |          0 | PRIMARY         |            1 | someid         | A         |     2811954 |     NULL | NULL   |      | BTREE    

上述两次之间的时间不到5秒.为什么每次调用show index时都会有这么大的差异？

这只发生在这个表中,我检查了几个较大的表,并且每次访问它们时都会显示相同的数字

仅供参考：

mysql> select count(*) from mybigusertable;
+----------+
| count(*) |
+----------+
|  3109320 | 
+----------+
1 row in set (4 min 34.00 sec)

几个问题：

>为什么基数变化如此之大,它真的重要吗？
> Optimize Table有多重要？它会使查询更快吗？

解决方法:

MySQL通过从索引中抽取随机页面来确定索引的基数.页面具有不同的记录数和分布.

对于基数不变的索引,索引很可能适合单个页面,或者页面具有均匀分布(例如,来自优化表).

如果计数差异很大,您可以考虑优化表以重新分配记录.这将有助于MySQL选择最佳索引.

标签：optimization,mysql,database,query-optimization,cardinality
来源： https://codeday.me/bug/20190709/1416276.html