Incompatible Change: The prepared_stmt_count system variable has been converted to the Prepared_stmt_count global status variable (viewable with the SHOW GLOBAL STATUS statement). (Bug#23159)

Incompatible Change: Previously, the DATE_FORMAT() function returned a binary string. Now it returns a string with a character set and collation given by character_set_connection and collation_connection so that it can return month and weekday names containing non-ASCII characters. (Bug#22646)

Security Fix: The requirement of the DROP privilege for RENAME TABLE was not enforced. (Bug#27515, CVE-2007-2691)

Incompatible Change: INSERT DELAYED statements are not supported for MERGE tables, but the MERGE storage engine was not rejecting such statements, resulting in table corruption. Applications previously using INSERT DELAYED into MERGE table will break when upgrading to versions with this fix. To avoid the problem, remove DELAYED from such statements. (Bug#26464)

Incompatible Change: For ENUM columns that had enumeration values containing commas, the commas were mapped to 0xff internally. However, this rendered the commas indistinguishable from true 0xff characters in the values. This no longer occurs. However, the fix requires that you dump and reload any tables that have ENUM columns containing any true 0xff values. Dump the tables using mysqldump with the current server before upgrading from a version of MySQL 4.1 older than 4.1.23 to version 4.1.23 or newer. (Bug#24660)