Primary Key and Secondary Key Indexes

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Primary Key and Secondary Key Indexes
Historically, the MySQL server and InnoDB have each kept their own metadata about table and
index structures. The MySQL server stores this information in .frm files that are not protected by a
transactional mechanism, while InnoDB has its own data dictionary as part of the system tablespace. If
a DDL operation was interrupted by a crash or other unexpected event partway through, the metadata
could be left inconsistent between these two locations, causing problems such as startup errors or
inability to access the table that was being altered. Now that InnoDB is the default storage engine,
addressing such issues is a high priority. These enhancements to DDL operations reduce the window
of opportunity for such issues to occur.
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