MySQL 5.0 が正式リリースされたことで、、バージョン 5.1 の開発に本格的に取り組み始めたようです。
以前より bitkeeper （ソースコード管理システム）上では頻繁に開発されていた、MySQL 5.1 シリーズの最初のバージョンがリリースされました。
Alpha 版としてのリリースですので、まだまだおかしな挙動をする点などあると思いますが、ご関心のあるかたはぜひお試しの上、メーリングリストなどに情報をお寄せいただければと思います。


ダウンロードはこちら（あるいはミラーサイトをご利用ください）：
http://dev.mysql.com/downloads/mysql/5.1.html

不具合報告などのフィードバックはこちら：
http://bugs.mysql.com/


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MySQL5.0 と比べて以下の変更があります。


- Partitioning, which allows distributing portions of individual tables across a filesystem, according to rules which can be set when the table is created. In effect, different portions of a table are stored as separate tables in different locations - but from the user point-of-view, the partitioned table is still a single table. See http://dev.mysql.com/doc/refman/5.1/en/partitioning.html for further information on this functionality. (Author: Mikael Ronström)

- MySQL 5.1 will add support for a very flexible plugin API, that will allow the loading and unloading of various components at runtime, without restarting the server. While the work on this is not finished yet, Pluggable Full-Text parsers are a first step in this direction. This allows the user to implement his own input filter on the indexed text, enabling full-text search capability on arbitrary data like PDF files or other document formats. A pre-parser full-text plugin performs the actual parsing and extraction of the text and hands it over to the builtin MySQL full-text search. (Author: Sergey Vojtovich)

- The Instance Manager (IM) now has some additional functionality:
* SHOW LOG FILES - provides a listing of all log files used by the instance
* SHOW LOG {ERROR | SLOW | GENERAL} size - retrieves a part of the specified log file
* SET instance_name.option_name=option_value - sets an option to the specified value and writes it to the config file See http://dev.mysql.com/doc/refman/5.1/en/instance-manager.html for more details on these new commands. (Author: Petr Chardin)

- Two new aggregate functions STDDEV_POP and STDDEV_SAMP for computing the population and sample standard deviation of expressions - see http://dev.mysql.com/doc/refman/5.1/en/group-by-functions.html for more info. (Author: Sergey Vojtovich)

- The performance of boolean full-text searches (using the "+" Operator) has been improved. See http://dev.mysql.com/doc/refman/5.1/en/fulltext-search.html for more details about full-text searching. (Author: Sergey Vojtovich)

- VARCHAR fields used in MySQL Cluster tables are now variable-sized ; that is, they now only allocate as much space as required to store the data. Previously, a VARCHAR(n) field allocated n+2 bytes (aligned to 4 bytes), regardless if the actual inserted value required that much space. (In other words, a VARCHAR column always required the same, fixed amount of storage as a CHAR column of the same size.)

- Added the "table_definition_cache" system variable. If you use a large number of tables, you can create a large table definition cache to speed up opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the normal table cache.

- The "table_cache" system variable was renamed to "table_open_cache". Any scripts that refer to "table_cache" should be updated to use the new name.