MySQL: Understanding the Limitations of ALTER IGNORE TABLE with InnoDB

The ALTER IGNORE TABLE command is intended to assist in removing duplicates from a MySQL table by adding a unique index. However, users have encountered unexpected errors when applying this command, such as the "Integrity constraint violation" error.

According to the MySQL documentation, the IGNORE keyword should allow for ignoring duplicate-key errors during the alteration process. However, an apparent bug in the InnoDB version of MySQL on certain releases has been identified. This bug prevents the IGNORE keyword from functioning as expected.

As a workaround, users can consider converting the table to MyISAM, adding the unique index with IGNORE, and then converting back to InnoDB. This process is outlined as follows:

ALTER TABLE table ENGINE MyISAM;
ALTER IGNORE TABLE table ADD UNIQUE INDEX dupidx (field);
ALTER TABLE table ENGINE InnoDB;

It's important to note that this solution may not be viable if the table has foreign key constraints, as these constraints will need to be removed prior to the conversion and then added back afterward.