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MySQL 5.5 Reference Manual
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Excerpts from this Manual Downgrade Notes

Before downgrading from MySQL 5.5, review the information in this section. Some items may require action before downgrading.

  • System Tables.  The mysql.proc.comment column definition changed between MySQL 5.1 and 5.5. After downgrading from 5.5 to 5.1, this table is seen as corrupt and in need of repair. Running mysql_upgrade from the version of MySQL to which you downgraded (as documented in the downgrade procedures) reverts the mysql.proc.comment column definition.

  • InnoDB.  MySQL 5.5 uses InnoDB Plugin as the built-in version of InnoDB. MySQL 5.1 includes InnoDB Plugin as of 5.1.38, but as an option that must be enabled explicitly. See the Release Notes for MySQL 5.1.38.

  • InnoDB.  In MySQL 5.5.14, the length limit for index prefix keys is increased from 767 bytes to 3072 bytes, for InnoDB tables using ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED. See Section, “Limits on InnoDB Tables” for details. If you downgrade from one of these releases or higher, to an earlier release with a lower length limit, the index prefix keys could be truncated at 767 bytes or the downgrade could fail. This issue could only occur if the configuration option innodb_large_prefix was enabled on the server being downgraded.

  • Tables partitioned by [LINEAR] KEY.  MySQL 5.5 implements new functions used to calculate row placement for tables partitioned by KEY and LINEAR KEY. Tables that were created using KEY or LINEAR KEY partitioning in MySQL 5.5 cannot be used by a MySQL 5.1 server. In MySQL 5.5.31 and later, you can downgrade such tables with ALTER TABLE ... PARTITION BY ALGORITHM=1 [LINEAR] KEY (...) to make them compatible with MySQL 5.1.

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