Instant DDL has been one of the most requested InnoDB features for a very long time. With ever larger and rapidly growing datasets the ability to do DDL instantly is a must have feature in any web scale database.  Developers constantly need to add new columns to meet the constantly changing business requirements.  …

The temporal datatypes ‘TIME/DATETIME/TIMESTAMP’ supports fractional values starting from MySQL 5.6.4 version. Hence the storage requirement and encoding differs in comparison to pre- MySQL 5.6.4 temporal datatypes.

As mentioned in the section ‘Upgrading from MySQL 5.5 to 5.6′,

http://dev.mysql.com/doc/refman/5.6/en/upgrading-from-previous-series.html

Once you have imported the pre-MySQL 5.6.4 tables, ‘ALTER TABLE’ allows creation of tables containing temporal columns of both mysql-5.5 and mysql-5.6 format.

As noted in the documentation, the disadvantages of having such tables are:

• Recreating tables having both 5.5 and 5.6 temporals becomes tedious process when the metadata file(.frm) for the table is unavailable. This would involve recreating the table having the old temporal columns using 5.5 server instance and then adding the 5.6 temporal columns using 5.6 server instance.
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