How to figure out size of Indexes in MySQL

I think this is what you're looking for.

show table status from [dbname]

http://dev.mysql.com/doc/refman/5.1/en/show-table-status.html

Extending Vajk Hermecz's answer.
This is how you can get all of the indexes size, in megabytes, without the PRIMARY (which is the table itself), ordered by size.

SELECT database_name, table_name, index_name,
ROUND(stat_value * @@innodb_page_size / 1024 / 1024, 2) size_in_mb
FROM mysql.innodb_index_stats
WHERE stat_name = 'size' AND index_name != 'PRIMARY'
ORDER BY size_in_mb DESC;

If you are using InnoDB tables, you can get the size for individual indexes from mysql.innodb_index_stats. The 'size' stat contains the answer, in pages, so you have to multiply it by the page-size, which is 16K by default.

select database_name, table_name, index_name, stat_value*@@innodb_page_size
from mysql.innodb_index_stats where stat_name='size';

Here's an adaption from some of the above to also give you the percentage of the total index for the table that each index has used, hopefully this will be useful for someone

select 
    database_name, 
    table_name, 
    index_name, 
    round((stat_value*@@innodb_page_size)/1024/1024, 2) SizeMB, 
    round(((100/(SELECT INDEX_LENGTH FROM INFORMATION_SCHEMA.TABLES t WHERE t.TABLE_NAME = iis.table_name and t.TABLE_SCHEMA = iis.database_name))*(stat_value*@@innodb_page_size)), 2) `Percentage`
from mysql.innodb_index_stats iis 
where stat_name='size' 
and table_name = 'TargetTable'
and database_name = 'targetDB'

Example output

database_name   table_name  index_name  SizeMB  Percentage
targetDB        TargetTable id          10      55.55
targetDB        TargetTable idLookup    5       27.77
targetDB        TargetTable idTest      3       16.66

Regards Liam

On MyISAM, each index block is 4 KB page filled up to fill_factor with index records, each being key length + 4 bytes long.

Fill factor is normally 2/3

As for InnoDB, the table is always clustered on the PRIMARY KEY, there is no separate PRIMARY KEY index

How to figure out size of Indexes in MySQL

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