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?
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.
索引键只允许我假设255个。
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 .
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