When downgrading to MySQL 5.1 from MySQL 5.5, you should keep in mind the following issues relating to features found in MySQL 5.5, but not in MySQL 5.1:
MySQL 5.5 uses
InnoDB Plugin as the
built-in version of
Plugin as of 5.1.38, but as an option that must
be enabled explicitly. See the
Notes for MySQL 5.1.38.
Tables partitioned by [LINEAR] KEY.
MySQL 5.5 implements new functions used to calculate row
placement for tables partitioned by
LINEAR KEY. Tables that were
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
TABLE ... PARTITION BY ALGORITHM=1 [LINEAR] KEY
(...) to make them compatible with MySQL 5.1.