怎么计算一个具体InnoDB的索引大小

一般情况下，我们看表信息可以用这个命令show table status：

mysql> show table status like 't'\G
*************************** 1. row ***************************
           Name: t
         Engine: InnoDB
        Version: 10
     Row_format: Compact
           Rows: 4186170
 Avg_row_length: 34
    Data_length: 143310848
Max_data_length: 0
   Index_length: 146030592
      Data_free: 6291456
 Auto_increment: NULL
    Create_time: 2014-02-04 15:40:54
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)

而这里的都是预估值，我们可以通过ANALYZE TABLE获取精确的值：

Data_length: 143310848,    136Mb  clustered index size.

Index_length: 146030592,  139Mb secondary index size.

比如这个有3个索引：1个自动生成的聚簇索引和2个普通索引：

 CREATE TABLE `t` (
  `a` smallint(6) DEFAULT NULL,
  `b` smallint(6) DEFAULT NULL,
  `c` smallint(6) DEFAULT NULL,
  KEY `a` (`a`),
  KEY `b` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

在5.6我们有更加明确的方式来知道索引的大小：

ANALYZE table t;
SELECT
       sum(stat_value) pages,
       index_name,
       sum(stat_value) * @@innodb_page_size size
FROM
       mysql.innodb_index_stats
WHERE
           table_name = 't'
       AND database_name = 'test'
       AND stat_description = 'Number of pages in the index'
GROUP BY
       index_name;

+-------+-----------------+-----------+
| pages | index_name      | size      |
+-------+-----------------+-----------+
|  8747 | GEN_CLUST_INDEX | 143310848 |
|  4456 | a               |  73007104 |
|  4457 | b               |  73023488 |
+-------+-----------------+-----------+
3 rows in set (0.00 sec)

那么在分区表中该如何获得索引的大小呢？

mysql> alter table t partition by key(c) partitions 4;
Query OK, 4194308 rows affected (44.03 sec)
Records: 4194308  Duplicates: 0  Warnings: 0

mysql> show create table t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE `t` (
  `a` smallint(6) DEFAULT NULL,
  `b` smallint(6) DEFAULT NULL,
  `c` smallint(6) DEFAULT NULL,
  KEY `a` (`a`),
  KEY `b` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
/*!50100 PARTITION BY KEY (c)
PARTITIONS 4 */
1 row in set (0.01 sec)

ANALYZE TABLE t;

SELECT
       sum(stat_value) pages,
       index_name,
       sum(stat_value) * @@innodb_page_size size
FROM
       mysql.innodb_index_stats
WHERE
           table_name LIKE 't#P%'
       AND database_name = 'test'
       AND stat_description LIKE 'Number of pages in the index'
GROUP BY
       index_name;

+-------+-----------------+-----------+
| pages | index_name      | size      |
+-------+-----------------+-----------+
|  8848 | GEN_CLUST_INDEX | 144965632 |
|  5004 | a               |  81985536 |
|  5004 | b               |  81985536 |
+-------+-----------------+-----------+
3 rows in set (0.00 sec)

mysql> SELECT
       sum(stat_value) pages,
       table_name part,
       index_name,
       sum(stat_value) * @@innodb_page_size size
FROM
       mysql.innodb_index_stats
WHERE
           table_name LIKE 't#P#%'
       AND database_name = 'test'
       AND stat_description LIKE 'Number of pages in the index'
GROUP BY
       table_name, index_name;

+-------+--------+-----------------+----------+
| pages | part   | index_name      | size     |
+-------+--------+-----------------+----------+
|  2212 | t#P#p0 | GEN_CLUST_INDEX | 36241408 |
|  1251 | t#P#p0 | a               | 20496384 |
|  1251 | t#P#p0 | b               | 20496384 |
|  2212 | t#P#p1 | GEN_CLUST_INDEX | 36241408 |
|  1251 | t#P#p1 | a               | 20496384 |
|  1251 | t#P#p1 | b               | 20496384 |
|  2212 | t#P#p2 | GEN_CLUST_INDEX | 36241408 |
|  1251 | t#P#p2 | a               | 20496384 |
|  1251 | t#P#p2 | b               | 20496384 |
|  2212 | t#P#p3 | GEN_CLUST_INDEX | 36241408 |
|  1251 | t#P#p3 | a               | 20496384 |
|  1251 | t#P#p3 | b               | 20496384 |
+-------+--------+-----------------+----------+
12 rows in set (0.00 sec)

参考资料：

http://aadant.com/blog/2014/02/04/how-to-calculate-a-specific-innodb-index-size/