Error Conditions for Online DDL
Here are the primary reasons why an online DDL operation could fail:
• If a LOCK clause specifies a low degree of locking (SHARED or NONE) that is not compatible with the
particular type of DDL operation.
• If a timeout occurs while waiting to get an exclusive lock on the table, which is needed briefly during
the initial and final phases of the DDL operation.
• If the tmpdir  file system runs out of disk space, while MySQL writes temporary sort files on
disk during index creation.
• If the ALTER TABLE takes so long, and concurrent DML modifies the table
so much, that the size of the temporary online long exceeds the value of the
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