迭代 1:

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    

上述两者之间的上述时间不到 5 秒。为什么每次调用显示索引时都会有如此大的差异？

这只发生在这张表上，我检查了几个更大的表，每次访问它们时它们都显示相同的数字

仅供引用:

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

几个问题:

1. 为什么基数变化如此之大，这真的很重要吗？
2. Optimize Table有多重要？它会使查询更快吗？

最佳答案

我认为问题源于 InnoDB 如何处理表元数据。

InnoDB 倾向于使用某种形式的搜索深度近似(由 optimizer_search_depth 决定)，这需要深入到猜测基数的索引。

尝试出发innodb_stats_on_metadata

SET GLOBAL innodb_stats_on_metadata = 0;

这将有助于更快地读取元数据并稳定查询执行计划。

更新 2012-03-06 11:55 EST

对 InnoDB 表执行 OPTIMIZE TABLE 是无用的，因为一旦您这样做以尝试编译索引统计信息，如果 innodb_stats_on_metadata 仍为 1，它往往会被再次读取。I wrote about this in the DBA StackExchange时间回到 2011 年 6 月。

更新 2012-03-06 11:59 EST

好的，既然您使用的是 MySQL 5.0.77，那么 OPTIMIZE TABLE 对 InnoDB 中的索引统计再生来说就毫无用处了。

OPTIMIZE TABLE 和 ANALYZE TABLE 仅适用于 MyISAM。

关于mysql - 显示表状态的疯狂基数计数，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9587131/