1.4. Overview of the MySQL Database Management System

• MySQL is a database management system.

  A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL Server. Since computers are very good at handling large amounts of data, database management systems play a central role in computing, as standalone utilities, or as parts of other applications.

• MySQL is a relational database management system.

  A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The SQL part of “MySQL” stands for “Structured Query Language.” SQL is the most common standardized language used to access databases and is defined by the ANSI/ISO SQL Standard. The SQL standard has been evolving since 1986 and several versions exist. In this manual, “SQL-92” refers to the standard released in 1992, “SQL:1999” refers to the standard released in 1999, and “SQL:2003” refers to the current version of the standard. We use the phrase “the SQL standard” to mean the current version of the SQL Standard at any time.

• MySQL software is Open Source.

  Open Source means that it is possible for anyone to use and modify the software. Anybody can download the MySQL software from the Internet and use it without paying anything. If you wish, you may study the source code and change it to suit your needs. The MySQL software uses the GPL (GNU General Public License), http://www.fsf.org/licenses/, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL code into a commercial application, you can buy a commercially licensed version from us. See the MySQL Licensing Overview for more information (http://www.mysql.com/company/legal/licensing/).

• The MySQL Database Server is very fast, reliable, and easy to use.

  If that is what you are looking for, you should give it a try. MySQL Server also has a practical set of features developed in close cooperation with our users. You can find a performance comparison of MySQL Server with other database managers on our benchmark page. See Section 7.1.4, “The MySQL Benchmark Suite”.

  MySQL Server was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Although under constant development, MySQL Server today offers a rich and useful set of functions. Its connectivity, speed, and security make MySQL Server highly suited for accessing databases on the Internet.

• MySQL Server works in client/server or embedded systems.

  The MySQL Database Software is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a wide range of application programming interfaces (APIs).

  We also provide MySQL Server as an embedded multi-threaded library that you can link into your application to get a smaller, faster, easier-to-manage standalone product.

• A large amount of contributed MySQL software is available.

  It is very likely that your favorite application or language supports the MySQL Database Server.

We started out with the intention of using the mSQL database system to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing, we came to the conclusion that mSQL was not fast enough or flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was designed to allow third-party code that was written for use with mSQL to be ported easily for use with MySQL.

The derivation of the name MySQL is not clear. Our base directory and a large number of our libraries and tools have had the prefix “my” for well over 10 years. However, co-founder Monty Widenius's daughter is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.

The name of the MySQL Dolphin (our logo) is “Sakila,” which was chosen by the founders of MySQL AB from a huge list of names suggested by users in our “Name the Dolphin” contest. The winning name was submitted by Ambrose Twebaze, an Open Source software developer from Swaziland, Africa. According to Ambrose, the feminine name Sakila has its roots in SiSwati, the local language of Swaziland. Sakila is also the name of a town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.

The following list describes some of the important characteristics of the MySQL Database Software. See also Section 1.6, “MySQL Development Roadmap”, for more information about current and upcoming features.

Internals and Portability:

Data Types:

Statements and Functions:

Security:

Scalability and Limits:

Connectivity:

Localization:

Clients and Tools:

This section addresses the questions, “How stable is MySQL Server?” and, “Can I depend on MySQL Server in this project?” We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing lists, which are very active in identifying problems as well as reporting types of use.

The original code stems back to the early 1980s. It provides a stable code base, and the ISAM table format used by the original storage engine remains backward-compatible. At TcX, the predecessor of MySQL AB, MySQL code has worked in projects since mid-1996, without any problems. When the MySQL Database Software initially was released to a wider public, our new users quickly found some pieces of untested code. Each new release since then has had fewer portability problems, even though each new release has also had many new features.

Each release of the MySQL Server has been usable. Problems have occurred only when users try code from the “gray zones.” Naturally, new users don't know what the gray zones are; this section therefore attempts to document those areas that are currently known. The descriptions mostly deal with Versions 3.23 and later of MySQL Server. All known and reported bugs are fixed in the latest version, with the exception of those listed in the bugs section, which are design-related. See Section A.8, “Known Issues in MySQL”.

The MySQL Server design is multi-layered with independent modules. Some of the newer modules are listed here with an indication of how well-tested each of them is:

MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the MyISAM storage engine in MySQL 3.23, the maximum table size was increased to 65536 terabytes (256⁷ – 1 bytes). With this larger allowed table size, the maximum effective table size for MySQL databases is usually determined by operating system constraints on file sizes, not by MySQL internal limits.

The InnoDB storage engine maintains InnoDB tables within a tablespace that can be created from several files. This allows a table to exceed the maximum individual file size. The tablespace can include raw disk partitions, which allows extremely large tables. The maximum tablespace size is 64TB.

The following table lists some examples of operating system file-size limits. This is only a rough guide and is not intended to be definitive. For the most up-to-date information, be sure to check the documentation specific to your operating system.

On Linux 2.2, you can get MyISAM tables larger than 2GB in size by using the Large File Support (LFS) patch for the ext2 filesystem. On Linux 2.4, patches also exist for ReiserFS to get support for big files (up to 2TB). Most current Linux distributions are based on kernel 2.4 or higher and include all the required LFS patches. With JFS and XFS, petabyte and larger files are possible on Linux. However, the maximum available file size still depends on several factors, one of them being the filesystem used to store MySQL tables.

For a detailed overview about LFS in Linux, have a look at Andreas Jaeger's Large File Support in Linux page at http://www.suse.de/~aj/linux_lfs.html.

Windows users please note: FAT and VFAT (FAT32) are not considered suitable for production use with MySQL. Use NTFS instead.

By default, MySQL creates MyISAM tables with an internal structure that allows a maximum size of about 4GB. You can check the maximum table size for a MyISAM table with the SHOW TABLE STATUS statement or with myisamchk -dv tbl_name. See Section 13.5.4, “SHOW Syntax”.

If you need a MyISAM table that is larger than 4GB and your operating system supports large files, the CREATE TABLE statement supports AVG_ROW_LENGTH and MAX_ROWS options. See Section 13.1.5, “CREATE TABLE Syntax”. You can also change these options with ALTER TABLE to increase a table's maximum allowable size after the table has been created. See Section 13.1.2, “ALTER TABLE Syntax”.

Other ways to work around file-size limits for MyISAM tables are as follows:

The MySQL Server itself has no problems with Year 2000 (Y2K) compliance:

The following simple demonstration illustrates that MySQL Server has no problems with DATE or DATETIME values through the year 9999, and no problems with TIMESTAMP values until after the year 2030:

mysql> DROP TABLE IF EXISTS y2k;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE y2k (date DATE,
    ->                   date_time DATETIME,
    ->                   time_stamp TIMESTAMP);
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO y2k VALUES
    -> ('1998-12-31','1998-12-31 23:59:59','1998-12-31 23:59:59'),
    -> ('1999-01-01','1999-01-01 00:00:00','1999-01-01 00:00:00'),
    -> ('1999-09-09','1999-09-09 23:59:59','1999-09-09 23:59:59'),
    -> ('2000-01-01','2000-01-01 00:00:00','2000-01-01 00:00:00'),
    -> ('2000-02-28','2000-02-28 00:00:00','2000-02-28 00:00:00'),
    -> ('2000-02-29','2000-02-29 00:00:00','2000-02-29 00:00:00'),
    -> ('2000-03-01','2000-03-01 00:00:00','2000-03-01 00:00:00'),
    -> ('2000-12-31','2000-12-31 23:59:59','2000-12-31 23:59:59'),
    -> ('2001-01-01','2001-01-01 00:00:00','2001-01-01 00:00:00'),
    -> ('2004-12-31','2004-12-31 23:59:59','2004-12-31 23:59:59'),
    -> ('2005-01-01','2005-01-01 00:00:00','2005-01-01 00:00:00'),
    -> ('2030-01-01','2030-01-01 00:00:00','2030-01-01 00:00:00'),
    -> ('2040-01-01','2040-01-01 00:00:00','2040-01-01 00:00:00'),
    -> ('9999-12-31','9999-12-31 23:59:59','9999-12-31 23:59:59');
Query OK, 14 rows affected, 2 warnings (0.00 sec)
Records: 14  Duplicates: 0  Warnings: 2

mysql> SELECT * FROM y2k;
+------------+---------------------+---------------------+
| date       | date_time           | time_stamp          |
+------------+---------------------+---------------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 1998-12-31 23:59:59 |
| 1999-01-01 | 1999-01-01 00:00:00 | 1999-01-01 00:00:00 |
| 1999-09-09 | 1999-09-09 23:59:59 | 1999-09-09 23:59:59 |
| 2000-01-01 | 2000-01-01 00:00:00 | 2000-01-01 00:00:00 |
| 2000-02-28 | 2000-02-28 00:00:00 | 2000-02-28 00:00:00 |
| 2000-02-29 | 2000-02-29 00:00:00 | 2000-02-29 00:00:00 |
| 2000-03-01 | 2000-03-01 00:00:00 | 2000-03-01 00:00:00 |
| 2000-12-31 | 2000-12-31 23:59:59 | 2000-12-31 23:59:59 |
| 2001-01-01 | 2001-01-01 00:00:00 | 2001-01-01 00:00:00 |
| 2004-12-31 | 2004-12-31 23:59:59 | 2004-12-31 23:59:59 |
| 2005-01-01 | 2005-01-01 00:00:00 | 2005-01-01 00:00:00 |
| 2030-01-01 | 2030-01-01 00:00:00 | 2030-01-01 00:00:00 |
| 2040-01-01 | 2040-01-01 00:00:00 | 0000-00-00 00:00:00 |
| 9999-12-31 | 9999-12-31 23:59:59 | 0000-00-00 00:00:00 |
+------------+---------------------+---------------------+
14 rows in set (0.00 sec)

The final two TIMESTAMP column values are zero because the year values (2040, 9999) exceed the TIMESTAMP maximum. The TIMESTAMP data type, which is used to store the current time, supports values that range from '1970-01-01 00:00:00' to '2030-01-01 00:00:00' on 32-bit machines (signed value). On 64-bit machines, TIMESTAMP handles values up to 2106 (unsigned value).

Although MySQL Server itself is Y2K-safe, you may run into problems if you use it with applications that are not Y2K-safe. For example, many old applications store or manipulate years using two-digit values (which are ambiguous) rather than four-digit values. This problem may be compounded by applications that use values such as 00 or 99 as “missing” value indicators. Unfortunately, these problems may be difficult to fix because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Thus, even though MySQL Server has no Y2K problems, it is the application's responsibility to provide unambiguous input. See Section 11.3.4, “Y2K Issues and Date Types”, for MySQL Server's rules for dealing with ambiguous date input data that contains two-digit year values.