Important Change: Incompatible Change: The FEDERATED storage engine is now disabled by default in binary distributions. The engine is still available and can be enabled by starting the server with the --federated option. (Bug#37069)

Incompatible Change: The engines column in the mysql.online_backup table has been renamed to drivers to better reflect its contents. (Bug#34965)

Incompatible Change: A change has been made to the way that the server handles prepared statements. This affects prepared statements processed at the SQL level (using the PREPARE statement) and those processed using the binary client-server protocol (using the mysql_stmt_prepare() C API function).

Previously, changes to metadata of tables or views referred to in a prepared statement could cause a server crash when the statement was next executed, or perhaps an error at execute time with a crash occurring later. For example, this could happen after dropping a table and recreating it with a different definition.

Now metadata changes to tables or views referred to by prepared statements are detected and cause automatic repreparation of the statement when it is next executed. Metadata changes occur for DDL statements such as those that create, drop, alter, rename, or truncate tables, or that analyze, optimize, or repair tables. Repreparation also occurs after referenced tables or views are flushed from the table definition cache, either implicitly to make room for new entries in the cache, or explicitly due to FLUSH TABLES.

Repreparation is automatic, but to the extent that it occurs, performance of prepared statements is diminished.

Table content changes (for example, with INSERT or UPDATE) do not cause repreparation, nor do SELECT statements.

An incompatibility with previous versions of MySQL is that a prepared statement may now return a different set of columns or different column types from one execution to the next. For example, if the prepared statement is SELECT * FROM t1, altering t1 to contain a different number of columns causes the next execution to return a number of columns different from the previous execution.

Older versions of the client library cannot handle this change in behavior. For applications that use prepared statements with the new server, an upgrade to the new client library is strongly recommended.

Along with this change to statement repreparation, the default value of the table_definition_cache system variable has been increased from 128 to 256. The purpose of this increase is to lessen the chance that prepared statements will need repreparation due to referred-to tables/views having been flushed from the cache to make room for new entries.

A new status variable, Com_stmt_reprepare, has been introduced to track the number of repreparations. (Bug#27420, Bug#27430, Bug#27690)

Important Change: Security Fix: Additional corrections were made for the symlink-related privilege problem originally addressed in MySQL 6.0.5. The original fix did not correctly handle the data directory pathname if it contained symlinked directories in its path, and the check was made only at table-creation time, not at table-opening time later. (Bug#32167, CVE-2008-2079)

See also Bug#39277.

Incompatible Change: SHOW STATUS took a lot of CPU time for calculating the value of the Innodb_buffer_pool_pages_latched status variable. Now this variable is calculated and included in the output of SHOW STATUS only when the UNIV_DEBUG symbol is defined at server build time. (Bug#36600)

Incompatible Change: Access privileges for several statements are more accurately checked:

(Bug#27145)

Incompatible Change: Certain characters were sorted incorrectly for the following collations: TILDE and GRAVE ACCENT in big5_chinese_ci; LATIN SMALL LETTER J in cp866_general_ci; TILDE in gb2312_chinese_ci; and TILDE in gbk_chinese_ci.

As a result of this fix, any indexes on columns that use these collations and contain the affected characters must be rebuilt when upgrading to 6.0.6 or higher. To do this, use ALTER TABLE to drop and re-add the indexes, or mysqldump to dump the affected tables and mysql to reload the dump file. (Bug#25420)

Incompatible Change: An additional correction to the original MySQL 6.0.4 fix was made to normalize directory names before adding them to the list of directories. This prevents /etc/ and /etc from being considered different, for example. (Bug#20748)

See also Bug#38180.