Functionality added or changed:
The KILL statement now takes CONNECTION and QUERY variants. The first is the same as KILL with no modifier (it kills a given connection thread). The second kills only the statement currently being executed by the connection.
Added TIMESTAMPADD() and TIMESTAMPDIFF() functions.
Added WEEK and QUARTER values as INTERVAL arguments for DATE_ADD() and DATE_SUB() functions.
New binary log format which enables replication of those session variables: SQL_MODE, SQL_AUTO_IS_NULL, FOREIGN_KEY_CHECKS (that one was already replicated since 4.0.14 but here it's done more efficiently: takes less space in the binary logs), UNIQUE_CHECKS. Other variables (like character sets, SQL_SELECT_LIMIT...) will be replicated in next 5.0.x releases.
Merge Index optimization for OR clauses. See OR optimizations.
Basic support for stored procedures (SQL:2003 style). See Stored Procedures.
Added SELECT INTO list_of_vars, which can be of mixed, that is, global and local type. See SELECT INTO Statement.
Easier replication upgrade (5.0.0 masters can read older binary logs, 5.0.0 slaves can read older relay logs; see Replication Upgrade for more details). The format of the binary log and relay log is changed compared to the one of MySQL 4.1 and older.
IMPORTANT NOTE: If you upgrade to InnoDB-4.1.1 or higher, you cannot downgrade to a version lower than 4.1.1 any more! That is because earlier versions of InnoDB are not aware of multiple tablespaces.
Bugs fixed: