MySQL executes alter table XX add column with duplicate entry error

mysql The version is 5.7.25-28-log, The storage engine of the table is InnoDB.
The implementation is as follows DDL:

alter table mytable 
add column ...( Some fields are omitted ),
add column price int NOT NULL DEFAULT '0' COMMENT ' Price ';

But reported an error

ERROR 1062 (23000): Duplicate entry '9277700--1' for key 'spu_sku'
Thu Sep  3 16:56:56 2020

This watch has UNIQUE KEY `spu_sku` (`spu`,`sku`), Will guarantee not to repeat , It shouldn't be Duplicate entry Error of ,
And when you query, you find that there is really only one record in the table spu yes 9277700,sku yes -1.

I checked the Internet and found that this is MySQL perform online DDL There will be problems . according to https://jira.mariadb.org/browse/MDEV-14535 The solution on this page .
perform DDL（ added ALGORITHM=COPY）:

alter table mytable 
add column ...( Some fields are omitted ),
add column price int NOT NULL DEFAULT '0' COMMENT ' Price '
ALGORITHM=COPY;

solve the problem , No more misstatements .

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Interested readers , You can refer to Mysql Official documents ：
https://dev.mysql.com/doc/refman/5.7/en/innodb-online-ddl-limitations.html