As a general rule, to upgrade from one release series to another, go to the next series rather than skipping a series. To upgrade from a release series previous to MySQL 5.1, upgrade to each successive release series in turn until you have reached MySQL 5.1, and then proceed with the upgrade to MySQL 5.5. For example, if you currently are running MySQL 5.0 and wish to upgrade to a newer series, upgrade to MySQL 5.1 first before upgrading to 5.5, and so forth. For information on upgrading to MySQL 5.1, see the MySQL 5.1 Reference Manual.

There is a special case for upgrading to MySQL 5.5, which is that there was a short-lived MySQL 5.4 development series. This series is no longer being worked on, but to accommodate users of both series, this section includes one subsection for users upgrading from MySQL 5.1 to 5.5 and another for users upgrading from MySQL 5.4 to 5.5.

To upgrade to MySQL 5.5, use the items in the following checklist as a guide:

For upgrades between versions of a MySQL release series that has reached General Availability status, you can move the MySQL format files and data files between different versions on systems with the same architecture. For upgrades to a version of a MySQL release series that is in development status, that is not necessarily true. Use of development releases is at your own risk.

If you are cautious about using new versions, you can always rename your old mysqld before installing a newer one. For example, if you are using a version of MySQL 5.1 and want to upgrade to 5.5, rename your current server from mysqld to mysqld-5.1. If your new mysqld then does something unexpected, you can simply shut it down and restart with your old mysqld.

If problems occur, such as that the new mysqld server does not start or that you cannot connect without a password, verify that you do not have an old my.cnf file from your previous installation. You can check this with the --print-defaults option (for example, mysqld --print-defaults). If this command displays anything other than the program name, you have an active my.cnf file that affects server or client operation.

If, after an upgrade, you experience problems with compiled client programs, such as Commands out of sync or unexpected core dumps, you probably have used old header or library files when compiling your programs. In this case, you should check the date for your mysql.h file and libmysqlclient.a library to verify that they are from the new MySQL distribution. If not, recompile your programs with the new headers and libraries. Recompilation might also be necessary for programs compiled against the shared client library if the library major version number has changed (for example from libmysqlclient.so.15 to libmysqlclient.so.16.

If your MySQL installation contains a large amount of data that might take a long time to convert after an in-place upgrade, you might find it useful to create a “dummy” database instance for assessing what conversions might be needed and the work involved to perform them. Make a copy of your MySQL instance that contains a full copy of the mysql database, plus all other databases without data. Run your upgrade procedure on this dummy instance to see what actions might be needed so that you can better evaluate the work involved when performing actual data conversion on your original database instance.

It is a good idea to rebuild and reinstall the Perl DBD::mysql module whenever you install a new release of MySQL. The same applies to other MySQL interfaces as well, such as PHP mysql extensions and the Python MySQLdb module.