Read all the items in the following sections to see whether any of them might affect your applications:

Note particularly any changes that are marked Known issue or Incompatible change. These incompatibilities with earlier versions of MySQL may require your attention before you upgrade. Our aim is to avoid these changes, but occasionally they are necessary to correct problems that would be worse than an incompatibility between releases. If any upgrade issue applicable to your installation involves an incompatibility that requires special handling, follow the instructions given in the incompatibility description. Often this will involve dumping and reloading tables, or use of a statement such as CHECK TABLE or REPAIR TABLE.

For dump and reload instructions, see Section 2.13.4, “Rebuilding or Repairing Tables or Indexes”. Any procedure that involves REPAIR TABLE with the USE_FRM option must be done before upgrading. Use of this statement with a version of MySQL different from the one used to create the table (that is, using it after upgrading) may damage the table. See Section 12.4.2.5, “REPAIR TABLE Syntax”.

Before upgrading to a new version of MySQL, Section 2.13.3, “Checking Whether Tables or Indexes Must Be Rebuilt”, to see whether changes to table formats or to character sets or collations were made between your current version of MySQL and the version to which you are upgrading. If so and these changes result in an incompatibility between MySQL versions, you will need to upgrade the affected tables using the instructions in Section 2.13.4, “Rebuilding or Repairing Tables or Indexes”.

After upgrading to a new version of MySQL, run mysql_upgrade (see Section 4.4.7, “mysql_upgrade — Check Tables for MySQL Upgrade”). This program checks your tables, and attempts to repair them if necessary. It also updates your grant tables to make sure that they have the current structure so that you can take advantage of any new capabilities. (Some releases of MySQL introduce changes to the structure of the grant tables to add new privileges or features.)

mysql_upgrade does not upgrade the contents of the help tables. For upgrade instructions, see Section 5.1.8, “Server-Side Help”.

If you run MySQL Server on Windows, see Section 2.3.7, “Upgrading MySQL on Windows”.

If you use replication, see Section 17.4.3, “Upgrading a Replication Setup”, for information on upgrading your replication setup.

Incompatible change: In MySQL 5.5, the server includes a plugin services interface that complements the plugin API. The services interface enables server functionality to be exposed as a “service” that plugins can access through a function-call interface. The libmysqlservices library provides access to the available services and dynamic plugins now must be linked against this library (use the -lmysqlservices flag). For an example showing what Makefile.am should look like, see Section 23.2.6, “MySQL Services for Plugins”.

Incompatible change: As of MySQL 5.5.7, the server requires that a new grant table, proxies_priv, be present in the mysql database. If you are upgrading from a previous MySQL release rather than performing a new installation, the server will find that this table is missing and exit during startup with the following message:

Table 'mysql.proxies_priv' doesn't exist

To create the proxies_priv table, start the server with the --skip-grant-tables option to cause it to skip the normal grant table checks, then run mysql_upgrade. For example:

shell> mysqld --skip-grant-tables &
shell> mysql_upgrade

Then stop the server and restart it normally.

You can specify other options on the mysqld command line if necessary. Alternatively, if your installation is configured so that the server normally reads options from an option file, use the --defaults-file option to specify the file (enter each command on a single line):

shell> mysqld --defaults-file=/usr/local/mysql/etc/my.cnf
         --skip-grant-tables &
shell> mysql_upgrade

With the --skip-grant-tables option, the server does no password or privilege checking, so any client can connect and effectively have all privilges. For additional security, use the --skip-networking option as well to prevent remote clients from connecting.

Incompatible change: As of MySQL 5.5.3, the Unicode implementation has been extended to provide support for supplementary characters that lie outside the Basic Multilingual Plane (BMP). Noteworthy features:

In most respects, upgrading to MySQL 5.5 should present few problems with regard to Unicode usage, although there are some potential areas of incompatibility. These are the primary areas of concern:

Consequently, if you want to upgrade tables from utf8 to utf8mb4 to take advantage of supplementary-character support, it may be necessary to change some column or index definitions.

For additional details about the new Unicode character sets and potential incompatibilities, see Section 9.1.10, “Unicode Support”, and Section 9.1.11, “Upgrading from Previous to Current Unicode Support”.

Incompatible change: As of MySQL 5.5.3, the server includes dtoa, a library for conversion between strings and numbers by David M. Gay. In MySQL, this library provides the basis for improved conversion between string or DECIMAL values and approximate-value (FLOAT/DOUBLE) numbers.

Because the conversions produced by this library differ in some cases from previous results, the potential exists for incompatibilities in applications that rely on previous results. For example, applications that depend on a specific exact result from previous conversions might need adjustment to accommodate additional precision.

For additional information about the properties of dtoa conversions, see Section 11.2, “Type Conversion in Expression Evaluation”.

Incompatible change: In MySQL 5.5, several changes were made regarding the language and character set of error messages:

For more information, see Section 9.2, “Setting the Error Message Language”, and Section 9.1.6, “Character Set for Error Messages”.

Incompatible change: Previously, the parser accepted an INTO clause in nested SELECT statements, which is invalid because such statements must return their results to the outer context. As of MySQL 5.5.3, this syntax is no longer permitted and statements that use it must be changed.

Some keywords may be reserved in MySQL 5.5 that were not reserved in MySQL 5.4. See Section 8.3, “Reserved Words”.