Upgrading from MySQL 5.1 to 5.5


It is good practice to back up your data before installing any new version of software. Although MySQL works very hard to ensure a high level of quality, you should protect your data by making a backup.

To upgrade to 5.5 from any previous version, MySQL recommends that you dump your tables with mysqldump before upgrading and reload the dump file after upgrading.

In general, you should do the following when upgrading from MySQL 5.1 to 5.5:

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.

The following lists describe changes that may affect applications and that you should watch out for when upgrading from MySQL 5.1 to 5.5.

Configuration Changes
  • Incompatible change: The InnoDB Plugin is included in MySQL 5.5 releases. It becomes the built-in version of InnoDB in MySQL Server, replacing the version previously included as the built-in InnoDB engine. InnoDB Plugin is also available in MySQL 5.1 as of 5.1.38, but it is an optional storage engine that must be enabled explicitly using two server options:


    If you were using InnoDB Plugin in MySQL 5.1 by means of those options, you must remove them after an upgrade to 5.5 or the server will fail to start.

    In addition, in InnoDB Plugin, the innodb_file_io_threads system variable has been removed and replaced with innodb_read_io_threads and innodb_write_io_threads. If you upgrade from MySQL 5.1 to MySQL 5.5 and previously explicitly set innodb_file_io_threads at server startup, you must change your configuration. Either remove any reference to innodb_file_io_threads or replace it with references to innodb_read_io_threads and innodb_write_io_threads.

  • 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”.

Server Changes
  • 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 to 5.5.7 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.


    This problem is fixed in MySQL 5.5.8; the server treats a missing proxies_priv table as equivalent to an empty table. However, after starting the server, you should still run mysql_upgrade to create the table.

  • Incompatible change: As of MySQL 5.5.7, InnoDB always uses the fast truncation technique, equivalent to DROP TABLE and CREATE TABLE. It no longer performs a row-by-row delete for tables with parent-child foreign key relationships. TRUNCATE TABLE returns an error for such tables. Modify your SQL to issue DELETE FROM table_name for such tables instead.

  • Incompatible change: Prior to MySQL 5.5.7, if you flushed the logs using FLUSH LOGS or mysqladmin flush-logs and mysqld was writing the error log to a file (for example, if it was started with the --log-error option), it renames the current log file with the suffix -old, then created a new empty log file. This had the problem that a second log-flushing operation thus caused the original error log file to be lost unless you saved it under a different name. For example, you could use the following commands to save the file:

    shell> mysqladmin flush-logs
    shell> mv host_name.err-old backup-directory

    To avoid the preceding file-loss problem, no renaming occurs as of MySQL 5.5.7; the server merely closes and reopens the log file. To rename the file, you can do so manually before flushing. Then flushing the logs reopens a new file with the original file name. For example, you can rename the file and create a new one using the following commands:

    shell> mv host_name.err host_name.err-old
    shell> mysqladmin flush-logs
    shell> mv host_name.err-old backup-directory
  • Incompatible change: As of MySQL 5.5.6, handling of CREATE TABLE IF NOT EXISTS ... SELECT statements has been changed for the case that the destination table already exists:

    • Previously, for CREATE TABLE IF NOT EXISTS ... SELECT, MySQL produced a warning that the table exists, but inserted the rows and wrote the statement to the binary log anyway. By contrast, CREATE TABLE ... SELECT (without IF NOT EXISTS) failed with an error, but MySQL inserted no rows and did not write the statement to the binary log.

    • MySQL now handles both statements the same way when the destination table exists, in that neither statement inserts rows or is written to the binary log. The difference between them is that MySQL produces a warning when IF NOT EXISTS is present and an error when it is not.

    This change in handling of IF NOT EXISTS results in an incompatibility for statement-based replication from a MySQL 5.1 master with the original behavior and a MySQL 5.5 slave with the new behavior. Suppose that CREATE TABLE IF NOT EXISTS ... SELECT is executed on the master and the destination table exists. The result is that rows are inserted on the master but not on the slave. (Row-based replication does not have this problem.)

    To address this issue, statement-based binary logging for CREATE TABLE IF NOT EXISTS ... SELECT is changed in MySQL 5.1 as of 5.1.51:

    This change provides forward compatibility for statement-based replication from MySQL 5.1 to 5.5 because when the destination table exists, the rows will be inserted on both the master and slave. To take advantage of this compatibility measure, the 5.1 server must be at least 5.1.51 and the 5.5 server must be at least 5.5.6.

    To upgrade an existing 5.1-to-5.5 replication scenario, upgrade the master first to 5.1.51 or higher. Note that this differs from the usual replication upgrade advice of upgrading the slave first.

    A workaround for applications that wish to achieve the original effect (rows inserted regardless of whether the destination table exists) is to use CREATE TABLE IF NOT EXISTS and INSERT ... SELECT statements rather than CREATE TABLE IF NOT EXISTS ... SELECT statements.

    Along with the change just described, the following related change was made: Previously, if an existing view was named as the destination table for CREATE TABLE IF NOT EXISTS ... SELECT, rows were inserted into the underlying base table and the statement was written to the binary log. As of MySQL 5.1.51 and 5.5.6, nothing is inserted or logged.

  • Incompatible change: Prior to MySQL 5.5.6, if the server was started with character_set_server set to utf16, it crashed during full-text stopword initialization. Now the stopword file is loaded and searched using latin1 if character_set_server is ucs2, utf16, or utf32. If any table was created with FULLTEXT indexes while the server character set was ucs2, utf16, or utf32, it should be repaired using this statement:

    REPAIR TABLE tbl_name QUICK;
  • Incompatible change: As of MySQL 5.5.5, all numeric operators and functions on integer, floating-point and DECIMAL values throw an “out of range” error (ER_DATA_OUT_OF_RANGE) rather than returning an incorrect value or NULL, when the result is out of the supported range for the corresponding data type. See Section 10.6, “Out-of-Range and Overflow Handling”.

  • 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:

    • utf16 and utf32 character sets have been added. These correspond to the UTF-16 and UTF-32 encodings of the Unicode character set, and they both support supplementary characters.

    • The utf8mb4 character set has been added. This is similar to utf8, but its encoding allows up to four bytes per character to enable support for supplementary characters.

    • The ucs2 character set is essentially unchanged except for the inclusion of some newer BMP characters.

    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:

    • For the variable-length character data types (VARCHAR and the TEXT types), the maximum length in characters is less for utf8mb4 columns than for utf8 columns.

    • For all character data types (CHAR, VARCHAR, and the TEXT types), the maximum number of characters that can be indexed is less for utf8mb4 columns than for utf8 columns.

    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:

    • The --language option for specifying the directory for the error message file is now deprecated. The new --lc-messages-dir and --lc-messages options should be used instead, and --language is handled as an alias for --lc-messages-dir.

    • The language system variable has been removed and replaced with the new lc_messages_dir and lc_messages system variables. lc_messages_dir has only a global value and is read only. lc_messages has global and session values and can be modified at runtime, so the error message language can be changed while the server is running, and individual clients each can have a different error message language by changing their session lc_messages value to a different locale name.

    • Error messages previously were constructed in a mix of character sets. This issue is resolved by constructing error messages internally within the server using UTF-8 and returning them to the client in the character set specified by the character_set_results system variable. The content of error messages therefore may in some cases differ from the messags returned previously.

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

SQL Changes
  • 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.

  • Incompatible change: In MySQL 5.5.3, several changes were made to alias resolution in multiple-table DELETE statements so that it is no longer possible to have inconsistent or ambiguous table aliases.

    • In MySQL 5.1.23, alias declarations outside the table_references part of the statement were disallowed for the USING variant of multiple-table DELETE syntax, to reduce the possibility of ambiguous aliases that could lead to ambiguous statements that have unexpected results such as deleting rows from the wrong table.

      As of MySQL 5.5.3, alias declarations outside table_references are disallowed for all multiple-table DELETE statements. Alias declarations are permitted only in the table_references part.


      DELETE FROM t1 AS a2 USING t1 AS a1 INNER JOIN t2 AS a2;
      DELETE t1 AS a2 FROM t1 AS a1 INNER JOIN t2 AS a2;


      DELETE FROM t1 USING t1 AS a1 INNER JOIN t2 AS a2;
      DELETE t1 FROM t1 AS a1 INNER JOIN t2 AS a2;
    • Previously, for alias references in the list of tables from which to delete rows in a multiple-table delete, the default database is used unless one is specified explicitly. For example, if the default database is db1, the following statement does not work because the unqualified alias reference a2 is interpreted as having a database of db1:

      DELETE a1, a2 FROM db1.t1 AS a1 INNER JOIN db2.t2 AS a2
      WHERE a1.id=a2.id;

      To correctly match an alias that refers to a table outside the default database, you must explicitly qualify the reference with the name of the proper database:

      DELETE a1, db2.a2 FROM db1.t1 AS a1 INNER JOIN db2.t2 AS a2
      WHERE a1.id=a2.id;

      As of MySQL 5.5.3, alias resolution does not require qualification and alias references should not be qualified with the database name. Qualified names are interpreted as referring to tables, not aliases.

    Statements containing alias constructs that are no longer permitted must be rewritten.

  • Some keywords may be reserved in MySQL 5.5 that were not reserved in MySQL 5.1. See Section 8.3, “Reserved Words”.

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