MySQL MyISAM - not able to add a new column to table

Question

I have the following table:

mysql> show create table keyword_links\G
*************************** 1. row ***************************
       Table: keyword_links
Create Table: CREATE TABLE `keyword_links` (
  `keyword_id` int(11) NOT NULL AUTO_INCREMENT,
  `keyword` tinytext NOT NULL,
  `link` tinytext,
  `weight` smallint(6) NOT NULL DEFAULT '0',
  `class_id` smallint(6) NOT NULL DEFAULT '0',
  `category_id` smallint(6) NOT NULL DEFAULT '0',
  `timestamp` timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`keyword_id`),
  KEY `class_id` (`class_id`),
  KEY `category_id` (`category_id`),
  KEY `idx_keyword` (`keyword`(333))
) ENGINE=MyISAM AUTO_INCREMENT=5082 DEFAULT CHARSET=utf8

to which I am trying to add a new column, which is failing:

mysql> ALTER TABLE keyword_links ADD COLUMN list_id INT UNSIGNED NOT NULL DEFAULT 0;
ERROR 1170 (42000): BLOB/TEXT column 'keyword' used in key specification without a key length

The index on keyword column does have a key length of 333, so why is it failing and how to fix it?

UPDATE

I tried reducing the size of the index on the keyword column from 333 to 255 and now I am able to add the new column successfully:

ALTER TABLE keyword_links DROP INDEX idx_keyword;
CREATE INDEX index_keyword ON keyword_links (keyword(255));
ALTER TABLE keyword_links ADD COLUMN list_id INT UNSIGNED NOT NULL DEFAULT 0;

But I would still like to know what's going on.

Answer 1

This is because of myisam's index's length limit of 1000 bytes (see this link)

Please note that this is all about bytes, and that a character might take 2, 3 or even 4 bytes depending on your encoding, and also that an INT will always be 4 bytes.