1.5 How stable is MySQL?

This section addresses the questions, ``how stable is MySQL?'' and, ``can I depend on MySQL in this project?'' Here we will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs).

At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of ``untested code'' that were quickly found by the new users who made queries in a manner different than our own. Each new release has had fewer portability problems than the previous one (even though each has had many new features), and we hope that it will be possible to label one of the next releases ``stable''.

Each release of MySQL has been usable and there have been problems only when users start to use code from ``the gray zones''. Naturally, outside users can't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions deal with the 3.22.x version of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the bugs section, which are things that are ``design''-related. E Known errors and design deficiencies in MySQL.

MySQL is written in multiple layers and different independent modules. These modules are listed below with an indication of how well-tested each of them is:

The ISAM table handler -- Stable

This manages storage and retrieval of all data in MySQL 3.22 and earlier versions. In all MySQL releases there hasn't been a single (reported) bug in this code. The only known way to get a corrupted table is to kill the server in the middle of an update. Even that is unlikely to destroy any data beyond rescue, because all data are flushed to disk between each query. There hasn't been a single bug report about lost data because of bugs in MySQL, either.

The MyISAM table handler -- Beta

This is new in MySQL 3.23. It's largely based on the ISAM table code but has a lot of new very useful features.

The parser and lexical analyser -- Stable

There hasn't been a single reported bug in this system for a long time.

The C client code -- Stable

No known problems. In early 3.20 releases, there were some limitations in the send/receive buffer size. As of 3.21.x, the buffer size is now dynamic up to a default of 24M.

Standard client programs -- Stable

These include mysql, mysqladmin and mysqlshow, mysqldump, and mysqlimport.

Basic SQL -- Stable

The basic SQL function system and string classes and dynamic memory handling. Not a single reported bug in this system.

Query optimizer -- Stable

Range optimizer -- Gamma

Join optimizer -- Stable

Locking -- Gamma

This is very system-dependent. On some systems there are big problems using standard OS locking (fcntl()). In these cases, you should run the MySQL daemon with the --skip-locking flag. Problems are known to occur on some Linux systems and on SunOS when using NFS-mounted file systems.

Linux threads -- Gamma

The only problem found has been with the fcntl() call, which is fixed by using the --skip-locking option to mysqld. Some people have reported lockup problems with the 0.5 release.

Solaris 2.5+ pthreads -- Stable

We use this for all our production work.

MIT-pthreads (Other systems) -- Gamma

There have been no reported bugs since 3.20.15 and no known bugs since 3.20.16. On some systems, there is a ``misfeature'' where some operations are quite slow (a 1/20 second sleep is done between each query). Of course, MIT-pthreads may slow down everything a bit, but index-based SELECT statements are usually done in one time frame so there shouldn't be a mutex locking/thread juggling.

Other thread implementions -- Alpha - Beta

The ports to other systems are still very new and may have bugs, possibly in MySQL, but most often in the thread implementation itself.

LOAD DATA ..., INSERT ... SELECT -- Stable

Some people have thought they have found bugs here, but these usually have turned out to be misunderstandings. Please check the manual before reporting problems!

ALTER TABLE -- Stable

Small changes in 3.22.12.

DBD -- Stable

Now maintained by Jochen Wiedmann

mysqlaccess -- Stable

Written and maintained by Yves Carlier

GRANT -- Gamma

Big changes made in MySQL 3.22.12.

MyODBC (uses ODBC SDK 2.5) -- Gamma

It seems to work well with some programs.

TcX provides email support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.