Important Change: The --skip-thread-priority option is now deprecated such that the server will not change the thread priorities by default. Giving threads different priorities might yield marginal improvements in some platforms (where it actually works), but it might instead cause significant degradation depending on the thread count and number of processors. Meddling with the thread priorities is a not a safe bet as it is very dependent on the behavior of the CPU scheduler and system where MySQL is being run. (Bug #35164, Bug #37536)

Important Change: The --log option now is deprecated and will be removed (along with the log system variable) in the future. Instead, use the --general_log option to enable the general query log and the --general_log_file=file_name option to set the general query log file name. The values of these options are available in the general_log and general_log_file system variables, which can be changed at runtime.

Similar changes were made for the --log-slow-queries option and log_slow_queries system variable. You should use the --slow_query_log and --slow_query_log_file=file_name options instead (and the slow_query_log and slow_query_log_file system variables).

The BUILD/compile-solaris-* scripts now compile MySQL with the mtmalloc library rather than malloc. (Bug #38727)

Incompatible Change; Replication: The default binary logging mode has been changed from MIXED to STATEMENT for compatibility with MySQL 5.0. (Bug #39812)

Incompatible Change: CHECK TABLE ... FOR UPGRADE did not check for incompatible collation changes made in MySQL 5.1.21 (Bug #29499) and 5.1.23 (Bug #27562, Bug #29461). This also affects mysqlcheck and mysql_upgrade, which cause that statement to be executed. See Checking Whether Tables or Indexes Must Be Rebuilt. (Bug #39585)

References: See also Bug #40984.

Incompatible Change: In connection with view creation, the server created arc directories inside database directories and maintained useless copies of .frm files there. Creation and renaming procedures of those copies as well as creation of arc directories has been discontinued.

This change does cause a problem when downgrading to older server versions which manifests itself under these circumstances:

As a workaround to avoid this problem, use either of these approaches:

The downgrade problem introduced by the fix for this bug has been addressed as Bug #40021. (Bug #17823)

Important Change; Replication: The SUPER privilege is now required to change the session value of binlog_format as well as its global value. For more information about binlog_format, see Replication Formats. (Bug #39106)

Partitioning; Replication: Replication to partitioned MyISAM tables could be slow with row-based binary logging. (Bug #35843)

Partitioning: Using INSERT ... SELECT to insert records into a partitioned MyISAM table could fail if some partitions were empty and others are not. (Bug #38005)

Partitioning: If an error occurred when evaluating a column of a partitioned table for the partitioning function, the row could be inserted anyway. (Bug #38083)

Partitioning: Ordered range scans on partitioned tables were not always handled correctly. In some cases this caused some rows to be returned twice. The same issue also caused GROUP BY query results to be aggregated incorrectly. (Bug #30573, Bug #33257, Bug #33555)

Replication: Server code used in binary logging could in some cases be invoked even though binary logging was not actually enabled, leading to asserts and other server errors. (Bug #38798)

Replication: Replication filtering rules were inappropiately applied when executing BINLOG pseudo-queries. One way in which this problem showed itself was that, when replaying a binary log with mysqlbinlog, RBR events were sometimes not executed if the --replicate-do-db option was specified. Now replication rules are applied only to those events executed by the slave SQL thread. (Bug #36099)

Replication: In some cases, a replication master sent a special event to a reconnecting slave to keep the slave's temporary tables, but they still had references to the “old” slave SQL thread and used them to access that thread's data. (Bug #38269)

Replication: For a CREATE TABLE ... SELECT statement that creates a table in a database other than the current one, the table could be created in the wrong database on replication slaves if row-based binary logging is used. (Bug #34707)

Replication: Replication of BLACKHOLE tables did not work with row-based binary logging. (Bug #38360)

Replication: A statement did not always commit or roll back correctly when the server was shut down; the error could be triggered by having a failing UPDATE or INSERT statement on a transactional table, causing an implicit rollback. (Bug #32709)

References: See also Bug #38262.

The Sun Studio compiler failed to build debug versions of the server due to use of features specific to gcc. (Bug #39451)

The combination of a subquery with a GROUP BY, an aggregate function calculated outside the subquery, and a GROUP BY on the outer SELECT could cause the server to crash. (Bug #37348)

In some cases, references to views were confused with references to anonymous tables and privilege checking was not performed. (Bug #36086)

The <=> operator could return incorrect results when comparing NULL to DATE, TIME, or DATETIME values. (Bug #37526)

Access checks were skipped for SHOW PROCEDURE STATUS and SHOW FUNCTION STATUS, which could lead to a server crash or insufficient access checks in subsequent statements. (Bug #37908)

When inserting a string into a duplicate-key error message, the server could improperly interpret the string, resulting in a crash. (Bug #38701)

Queries of the form SELECT ... REGEXP BINARY NULL could lead to a hung or crashed server. (Bug #39021)

Repeated CREATE TABLE ... SELECT statements, where the created table contained an AUTO_INCREMENT column, could lead to an assertion failure. (Bug #38821)

Statements of the form INSERT ... SELECT .. ON DUPLICATE KEY UPDATE col_name = DEFAULT could result in a server crash. (Bug #39002)

An uninitialized variable in the query profiling code was corrected (detected by Valgrind). (Bug #38560)

Dynamic plugins failed to load on i5/OS. (Bug #35743)

A server crash resulted from concurrent execution of a multiple-table UPDATE that used a NATURAL or USING join together with FLUSH TABLES WITH READ LOCK or ALTER TABLE for the table being updated. (Bug #38691)

Column names constructed due to wild-card expansion done inside a stored procedure could point to freed memory if the expansion was performed after the first call to the stored procedure. (Bug #38823)

A server crash resulted from execution of an UPDATE that used a derived table together with FLUSH TABLES. (Bug #38499)

The handlerton-to-plugin mapping implementation did not free handler plugin references when the plugin was uninstalled, resulting in a server crash after several install/uninstall cycles. Also, on Mac OS X, the server crashed when trying to access an EXAMPLE table after the EXAMPLE plugin was installed. (Bug #37958)

For deadlock between two transactions that required a timeout to resolve, all server tables became inaccessible for the duration of the deadlock. (Bug #38804)

Stored procedures involving substrings could crash the server on certain platforms due to invalid memory reads. (Bug #38469)

ALTER EVENT changed the PRESERVE attribute of an event even when PRESERVE was not specified in the statement. (Bug #35981)

The server crashed if an argument to a stored procedure was a subquery that returned more than one row. (Bug #37949)

References to local variables in stored procedures are replaced with NAME_CONST(name, value) when written to the binary log. However, an “illegal mix of collation” error might occur when executing the log contents if the value's collation differed from that of the variable. Now information about the variable collation is written as well. (Bug #39182)

For a TIMESTAMP column in an InnoDB table, testing the column with multiple conditions in the WHERE clause caused a server crash. (Bug #39353)

With the PAD_CHAR_TO_FULL_LENGTH SQL mode enabled, a ucs2 CHAR column returned additional garbage after trailing space characters. (Bug #35720)

mysql_install_db failed on machines that had the host name set to localhost. (Bug #35754)

For crash reports on Windows, symbol names in stack traces were not correctly resolved. (Bug #35987)

The NO_BACKSLASH_ESCAPES SQL mode was ignored for LOAD DATA INFILE and SELECT INTO ... OUTFILE. The setting is taken into account now. (Bug #37114)

On ActiveState Perl, mysql-test-run.pl --start-and-exit started but did not exit. (Bug #38629)

Host name values in SQL statements were not being checked for '@', which is illegal according to RFC952. (Bug #35924)

When analyzing the possible index use cases, the server was incorrectly reusing an internal structure, leading to a server crash. (Bug #37943)

A race condition between threads sometimes caused unallocated memory to be addressed. (Bug #38692)

mysqld_safe would sometimes fail to remove the pid file for the old mysql process after a crash. As a result, the server would fail to start due to a false A mysqld process already exists... error. (Bug #11122)

mysqldump could fail to dump views containing a large number of columns. (Bug #31434)

The server could improperly type user-defined variables used in the select list of a query. (Bug #26020)

A trigger for an InnoDB table activating multiple times could lead to AUTO_INCREMENT gaps. (Bug #31612)

For access to the INFORMATION_SCHEMA.VIEWS table, the server did not check the SHOW VIEW and SELECT privileges, leading to inconsistency between output from that table and the SHOW CREATE VIEW statement. (Bug #22763)