Important Change: If you upgrade to MySQL 4.1.1 or higher, it is difficult to downgrade back to 4.0 or 4.1.0. That is because, for earlier versions, InnoDB is not aware of multiple tablespaces.

Replication: New binary log format that enables replication of these session variables: sql_mode, sql_auto_is_null, foreign_key_checks (which was replicated since 4.0.14, but here it is done more efficiently and takes less space in the binary logs), unique_checks. Other variables (like character sets, sql_select_limit, ...) will be replicated in upcoming 5.0.x releases.

Replication: Easier replication upgrade (5.0.0 masters can read older binary logs and 5.0.0 slaves can read older relay logs). See Replication Compatibility Between MySQL Versions, for more details). The format of the binary log and relay log is changed compared to that of MySQL 4.1 and older.

LOAD DATA INFILE causes an implicit commit.

Added SELECT ... INTO list_of_vars, which can be of mixed (that is, global and local) types. See SELECT ... INTO Syntax.

The precedence of the XOR operator now lies between OR and AND. Previously, XOR had the same precedence as OR.

Added WEEK and QUARTER values as INTERVAL arguments for the DATE_ADD() and DATE_SUB() functions.

For user-defined functions (UDFs), the UDF_ARGS structure now has attributes and attribute_lengths members that provide information about the argument names. UDF Argument Processing.

Implemented Index Merge optimization for OR clauses. See Index Merge Optimization.

The output of the SHOW BINLOG EVENTS statement has been modified. The Orig_log_pos column has been renamed to End_log_pos and now represents the offset of the last byte of the event, plus one.

Basic support for stored procedures and functions (SQL:2003 style). See Using Stored Routines (Procedures and Functions).

Added support for SUM(DISTINCT), MIN(DISTINCT), and MAX(DISTINCT).

The KILL statement now takes CONNECTION and QUERY modifiers. 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.