MDEV-26294 Duplicate entries in unique index not detected when changing collation
Problem: ======= ALTER TABLE in InnoDB fails to detect duplicate entries for the unique index when the character set or collation of an indexed column is changed in such a way that the character encoding is compatible with the old table definition. In this case, any secondary indexes on the changed columns would be rebuilt (DROP INDEX, ADD INDEX). Solution: ======== During ALTER TABLE, InnoDB keeps track of columns whose collation changed, and will fill in the correct metadata when sorting the index records, or applying changes from concurrent DML. This metadata will be allocated in the dict_index_t::heap of the being-created secondary indexes. The fix was developed by Thirunarayanan Balathandayuthapani and simplified by me.
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