alter ignore为啥不起作用

下面是官方对alter ignore add unique index的解释
IGNORE is a MySQL extension to standard SQL. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table or if warnings occur when strict mode is enabled. If IGNORE is not specified, the copy is aborted and rolled back if duplicate-key errors occur. If IGNORE is specified, only the first row is used of rows with duplicates on a unique key, The other conflicting rows are deleted. Incorrect values are truncated to the closest matching acceptable value.
但是实践中为什么不给力

提示什么，MYSQL什么版本

5.1之后的版本貌似不行