# Bugs Fixed
* InnoDB; Performance: Some data structures related to undo logging could be initialized unnecessarily during a query, although they were only needed under specific conditions.
* InnoDB; Performance: Optimized read operations for compressed tables by skipping redundant tests. The check for whether any related changes needed to be merged from the insert buffer was being called more often than necessary.
* InnoDB; Performance: Immediately after a table was created, queries against it would not use loose index scans. The issue went away following an ALTER TABLE on the table. The fix improves the accuracy of the index statistics gathered when the table is first created, and prevents the query plan from being changed by the ALTER TABLE statement.
* InnoDB; Partitioning: Previously, when attempting to optimize one or more partitions of a partitioned table that used a storage engine that does not support partition-level OPTIMIZE, such as InnoDB, MySQL reported Table does not support optimize, doing recreate + analyze instead, then re-created the entire table, but did not actually analyze it. Now in such cases, the warning message is, Table does not support optimize on partitions. All partitions will be rebuilt and analyzed. In addition, the entire table is analyzed after first being rebuilt.
* InnoDB: The status variable Innodb_buffer_pool_read_ahead_evicted could show an inaccurate value, higher than expected, because some pages in the buffer pool were incorrectly considered as being brought in by read-ahead requests.
* InnoDB: Creating an index on a CHAR column could fail for a table with a character set with varying length, such as UTF-8, if the table was created with the ROW_FORMAT=REDUNDANT clause.
* InnoDB: If the server crashed at a precise moment during an ALTER TABLE operation that rebuilt the clustered index for an InnoDB table, the original table could be inaccessible afterward. An example of such an operation is ALTER TABLE ... ADD PRIMARY KEY The fix preserves the original table if the server halts during this operation. You might still need to rename the .ibd file manually to restore the original table contents: in MySQL 5.6 and higher, rename from #sql-ib$new_table_id.ibd to table_name.ibd within the database directory; prior to MySQL 5.6, the temporary file to rename is table_name#1 or #2.
* InnoDB: An error at the filesystem level, such as too many open files, could cause an unhandled error during an ALTER TABLE operation. The error could be accompanied by Valgrind warnings, and by this assertion message: Assertion `! is_set()' failed. mysqld got signal 6 ;
* InnoDB: During shutdown, with the innodb_purge_threads configuration option set greater than 1, the server could halt prematurely with this error: mysqld got signal 11 A workaround was to increase innodb_log_file_size and set innodb_purge_threads=1. The fix was backported to MySQL 5.5 and 5.1, although those versions do not have the innodb_purge_threads configuration option so the error was unlikely to occur.
* InnoDB: The value of the innodb_version variable was not updated consistently for all server releases for the InnoDB Plugin in MySQL 5.1, and the integrated InnoDB component in MySQL 5.5, 5.6, and higher. Since InnoDB and MySQL Server development cycles are fully integrated and synchronized, now the value returned by the innodb_version variable is the same as for the version variable.
* Partitioning: When used with a table having multiple columns in its primary key, but partitioned by KEY using a column that was not part of the primary key as the partitioning column, a query using an aggregate function and DISTINCT such as SELECT SUM(DISTINCT pk_column_1) FROM table WHERE pk_column_2 = constant was not handled correctly.
* Replication: Repeated execution of CHANGE MASTER TO statements using invalid MASTER_LOG_POS values could lead to errors and possibly a crash on the slave. Now in such cases, the statement fails with a clear error message.
* Replication: If the disk becomes full while writing to the binary log, the server hangs until space is freed up manually. It was possible after this was done for the MySQL server to fail, due to an internal status value being set when not needed. Now in such cases, rather than trying to set this status, a warning is written in the error log instead.
* Microsoft Windows: Dynamic file names (with colons) are no longer allowed. Static file names using the Alternate Data Stream (ADS) NTFS functionality of Microsoft Windows may continue to be used.
* Directory name manipulation could result in stack overflow on Mac OS X and Windows.
* A buffer-handling problem in yaSSL was fixed.
* It was possible in theory for UpdateXML() to return NULL incorrectly.
* Metadata locking and table definition cache routines did not always check length of names passed to them.
* Enabling the query cache during high client contention could cause the server to exit.
* The server sometimes failed to respect MAX_CONNECTIONS_PER_HOUR limits on user connections.
* Passing an unknown time zone specification to CONVERT_TZ() resulted in a memory leak.
* For MEMORY tables with HASH indexes, DELETE sometimes failed to delete all applicable rows.
* mysqld_safe used the nonportable -e test construct.
* UNION type conversion could incorrectly turn unsigned values into signed values.
* During the startup process, mysqld could incorrectly remove the PID file of an already running mysqld.