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Around 1999–2000 a separate company named MySQL AB was established. It hired several developers and established a partnership with Sleepycat to provide an SQL interface for the Berkeley DB data files. Since Berkeley DB had transaction capabilities, this would give MySQL support for transactions, which it previously lacked.
After some changes in the code in preparation for integrating Berkeley DB, version 3.23 was released. Although the MySQL developers could never work out all the quirks of the Berkeley DB interface and the Berkeley DB tables were never stable, the effort was not wasted. As a result, MySQL source became equipped with hooks to add any type of storage engine, including a transactional one.
By April of 2000, with some encouragement and sponsorship from Slashdot, masterslave replication capability was added. The old nontransactional storage engine,ISAM, was reworked and released as MyISAM. Among a number of improvements, full-text search capabilities were now supported. A short-lived partnership with NuSphere to add Gemini, a transactional engine with row-level locking, ended in a lawsuit toward the end of 2001. However, around the same time, Heikki Tuuri approached MySQL AB with a proposal to integrate his own storage engine,
InnoDB, which was also capable of transactions and row-level locking.
Heikki’s contribution integrated much more smoothly with the new table handler interface already polished off by the Berkeley DB integration efforts. The MySQL/InnoDB combination became version 4.0, and was released as alpha in October of 2001. By early 2002 the MySQL/InnoDB combo was stable and instantly took MySQL to another level. Version 4.0 was finally declared production stable in March 2003.
It might be worthy of mention that the version number change was not caused by the addition of InnoDB. MySQL developers have always viewed InnoDB as an important addition, but by no means something that they completely depend on for success. Back then, and even now, the addition of a new storage engine is not likely to be celebrated with a version number change. In fact, compared to previous versions, not much was added in version 4.0. Perhaps the most significant addition was the query cache, which greatly improved performance of a large number of applications.
Replication code on the slave was rewritten to use two threads: one for network I/O from the master, and the other to process the updates. Some improvements were added to the optimizer. The client/server protocol became SSL-capable.Version 4.1 was released as alpha in April of 2003, and was declared beta in June of 2004. Unlike version 4.0, it added a number of significant improvements. Perhaps the most significant was subqueries, a feature long-awaited by many users. Spatial indexing support was added to the MyISAM storage engine. Unicode support was implemented. The client/server protocol saw a number of changes. It was made more secure against attacks, and supported prepared statements.
In parallel with the alpha version of 4.1, work progressed on yet another development branch: version 5.0, which would add stored procedures, server-side cursors, triggers, views, XA transactions, significant improvements in the query optimizer, and a number of other features. The decision to create a separate development branch was made because MySQL developers felt that it would take a long time to stabilize 4.1 if, on top of all the new features that they were adding to it, they had to deal with the stored procedures. Version 5.0 was finally released as alpha in December 2003. For a while this created quite a bit of confusion—there were two branches in the alpha stage. Eventually 4.1 stabilized (October 2004), and the confusion was resolved.
Version 5.0 stabilized a year later, in October of 2005. The first alpha release of 5.1 followed in November 2005, which added a number of improvements, some of which are table data partitioning, row-based replication, event scheduler, and a standardized plug-in API that facilitates the integration of new storage engines and other plug-ins.
At this point, MySQL is being actively developed. 5.0 is currently the stable version, while 5.1 is in beta and should soon become stable. New features at this point go into version 5.2.
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