OPTIMIZE TABLE will for MyISAM tables treat all NULL values as different when calculating cardinality. This helps in optimizing joins between tables where one of the tables has a lot of NULL values in a indexed column:

SELECT * from t1,t2 where t1.a=t2.key_with_a_lot_of_null;

Added join operator FORCE INDEX (key_list). This acts likes USE INDEX (key_list) but with the addition that a table scan is assumed to be VERY expensive. One bad thing with this is that it makes FORCE a reserved word.

Reset internal row buffer in MyISAM after each query. This will reduce memory in the case you have a lot of big blobs in a table.

A security patch in 4.0.8 causes the mysqld server to die if the remote hostname can't be resolved. This is now fixed.

Fixed crash when replication big LOAD DATA INFILE statement that caused log rotation.