Recently while we were building a slave with a newer version of MySQL 5.1 from an InnoDB Hot backup, the following error occurred when we ran "mysql_upgrade":mysql.time_zone OKmysql.time_zone_leap_second OKmysql.time_zone_name OKmysql.time_zone_transition OKmysql.time_zone_transition_type OKmysql.user OKRunning 'mysql_fix_privilege_tables'...ERROR 13 (HY000) at line 311: Can't get stat of './mysql/general_log.CSV' (Errcode: 2)ERROR 13 (HY000) at line 316: Can't get stat of './mysql/slow_log.CSV' (Errcode: 2)FATAL ERROR: Upgrade failedThe problem is that in MySQL 5.1, it is possible to log the slow query log and general log to tables in the "mysql" schema (source: Selecting General Query and Slow Query Log Output Destinations). These tables are  []

In two words: online operations. In a paragraph: Forget partitioning, row-based replication and events. The big reasons most people are going to salivate over 5.1, and probably start plans to upgrade now, are the online operations: \tonline ALTER TABLE for column rename, column default value change, and adding values to the end of an ENUM/SET\tOnline, table-based logging. No more need to restart your server to enable or change the general or slow query logs. You can have the standard file-based output or choose a table format...which you can query.

Sergey from the MySQL Optimizer team talks about recent developments in the MySQL Optimizer, including: What's new in MySQL 5.1 optimizer, Overview of known optimizer problems in MySQL 5.1, State of new optimizations in MySQL 6.0, Optimizer development beyond 6.0