MySQL 5.5.16

# Bugs Fixed
* InnoDB Storage Engine: The “random read-ahead” feature that was removed from the InnoDB Plugin is now available again. Because it is only helpful for certain workloads, it is turned off by default. To turn it on, enable the innodb_random_read_ahead configuration option. Because this feature can improve performance in some cases and reduce performance in others, before relying on this setting, benchmark both with and without the setting enabled.
* The metadata locking subsystem added too much overhead for INFORMATION_SCHEMA queries that were processed by opening only .frm or .TRG files and had to scan many tables. For example, SELECT COUNT(*) FROM INFORMATION_SCHEMA.TRIGGERS was affected.
* Compilation failed on Mac OS X 10.7 (Lion) with a warning: Implicit declaration of function 'pthread_init'
* With profiling disabled or not compiled in, set_thd_proc_info() unnecessarily checked file name lengths.
* A DBUG_ASSERT added by Bug #11792200 was overly aggressive in raising assertions.
* CHECK TABLE and REPAIR TABLE failed to find problems with MERGE tables that had underlying tables missing or with the wrong storage engine. Issues were reported only for the first underlying table.
* Replication: Processing of corrupted table map events could cause the server to crash. This was especially likely if the events mapped different tables to the same identifier, such as could happen due to Bug#56226. Now, before applying a table map event, the server checks whether the table has already been mapped with different settings, and if so, an error is raised and the slave SQL thread stops. If it has been mapped with the same settings, or if the table is set to be ignored by filtering rules, there is no change in behavior: the event is skipped and IDs are not checked.
* For a lower_case_table_names value of 1 or 2 and a database having a mixed-case name, calling a stored function using a fully qualified name including the database name failed.
* Previously, Performance Schema table columns that held byte counts were BIGINT UNSIGNED. These were changed to BIGINT (signed). This makes it easier to perform calculations that compute differences between columns.
* The embedded server crashed when argc = 0.
* The mysql_affected_rows() C API function returned 3 (instead of 2) for INSERT ... ON DUPLICATE KEY UPDATE statements where there was a duplicated key value.
* Upgrades using an RPM package recreated the test database, which is undesirable when the DBA had removed it.