This is duplicate of https://perconadev.atlassian.net/browse/PT-169#icft=PT-169

**Comment from Launchpad by: Hamoon Mohammadian Pour on: 29-06-2017 05:53:20

Thank you for testing.
I think the document should be updated and warned to prevent this happen.

**Comment from Launchpad by: Carlos Salguero on: 26-06-2017 17:45:55

To try to re-reproduce this error, I wrote this small Go program to load some sample data: https://gist.github.com/percona-csalguero/22457e02e46e10219ad0d8df892ce8e1

On my first try, I was unable to reproduce the problem, even when the program fails, no data is deleted:

Determining the method to update foreign keys...
2017-06-26T14:34:48 `test`.`user_comments`: 96850 rows; can use rebuild_constraints
2017-06-26T14:34:48 Analyzing new table...
2017-06-26T14:34:48 Swapping tables...
2017-06-26T14:34:48 Swapped original and new tables OK.
2017-06-26T14:34:48 Rebuilding foreign key constraints...
2017-06-26T14:34:48 Dropping triggers...
2017-06-26T14:34:48 Dropped triggers OK.
Altered `test`.`users` but there were errors or warnings.
Error updating foreign key constraints: 2017-06-26T14:34:48 DBD::mysql::db do failed: Failed to add the foreign key constaint. Missing index for constraint '_fk1' in the referenced table 'users' [for Statement "ALTER TABLE `test`.`user_comments` DROP FOREIGN KEY `fk1`, ADD CONSTRAINT `_fk1` FOREIGN KEY (`user_id`) REFERENCES `test`.`users` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION"] at bin/pt-online-schema-change line 10674.

As you can see in the log, osc chose rebuild_constraints method for FKs:
2017-06-26T14:34:48 `test`.`user_comments`: 96850 rows; can use rebuild_constraints

So I've updated my program to load 200K rows instead of 100K ( max_number = 1E5) and ran osc again:

Determining the method to update foreign keys...
2017-06-26T14:37:39 `test`.`user_comments`: too many rows: 184955; must use drop_swap
2017-06-26T14:37:39 Drop-swapping tables...
2017-06-26T14:37:39 Analyzing new table...
2017-06-26T14:37:39 Dropping triggers...
2017-06-26T14:37:39 Dropped triggers OK.
2017-06-26T14:37:39 Dropping new table...
2017-06-26T14:37:39 Dropped new table OK.
Altered `test`.`users` but there were errors or warnings.
Error updating foreign key constraints: 2017-06-26T14:37:39 DBD::mysql::db do failed: Error on rename of './test/_users_new' to './test/users' (errno: 150 "Foreign key constraint is incorrectly formed") [for Statement "RENAME TABLE `test`.`_users_new` TO `test`.`users`"] at bin/pt-online-schema-change line 10742.

This time the program failed and since there are too many rows, osc chose drop_swap to rebuild constraints, so to be able to reproduce this issue, --alter-foreign-keys-method should be drop_swap.

**Comment from Launchpad by: Carlos Salguero on: 26-06-2017 15:23:36

I've verified it fails but if you don't force set-vars=foreign_key_checks=0, all data is preserved.

**Comment from Launchpad by: Hamoon Mohammadian Pour on: 23-06-2017 12:11:01

Hi Carlos,
Look at post number 0 and number 2.
I explained