How Crash Recovery Works with Online DDL
Although no data is lost if the server crashes while an ALTER TABLE statement is executing, the crash
recovery process is different for clustered indexes and secondary indexes.
If the server crashes while creating an InnoDB secondary index, upon recovery, MySQL drops any
partially created indexes. You must re-run the ALTER TABLE or CREATE INDEX statement.
When a crash occurs during the creation of an InnoDB clustered index, recovery is more complicated,
because the data in the table must be copied to an entirely new clustered index. Remember that all
InnoDB tables are stored as clustered indexes. In the following discussion, we use the word table and
clustered index interchangeably.
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