MySQL is a relational database management system (RDBMS) that runs as a server providing multi-user access to a number of databases. MySQL is officially pronounced /maɪˌɛskjuːˈɛl/ ("My S-Q-L"), but is often pronounced /maɪˈsiːkwəl/ ("My Sequel"). It is named for original developer Michael Widenius's daughter My.
The MySQL development project has made its source code available under the terms of the GNU General Public License, as well as under a variety of proprietary agreements. MySQL is owned and sponsored by a single for-profit firm, the Swedish company MySQL AB, now owned by Sun Microsystems, a subsidiary of Oracle Corporation
New in MySQL 5.5
It's been a busy year for MySQL. Perhaps you've heard. Here are some recent improvements to the speed, scalability, and user-friendliness of the MySQL database and the InnoDB storage engine that we think deserve their own headlines. Now is a great time to beta test the 5.5 release and give feedback to the MySQL engineering team. Improved Performance and Scalability
InnoDB Becomes Default Storage Engine
MySQL sometimes gets knocked about features such as as ACID-compliant transactions, foreign key support, and crash recovery. These features are strongest in the InnoDB storage engine, but MyISAM has always been the default, so new users could get the wrong impression. Starting in MySQL 5.5, InnoDB is the default storage engine, so that everyone can see this reliability and stability out of the box. As a bonus, the level of InnoDB in MySQL 5.5 is InnoDB 1.1, a rearchitected InnoDB with many performance and scalability features over and above the built-in InnoDB in 5.1 and before. (Since we are unifying the InnoDB within MySQL using the best and fastest technology, we are phasing out the "Plugin" versus "Built-In" distinction; MySQL 5.5 comes with the latest and greatest InnoDB 1.1.)
Better Metadata Locking within Transactions
If a table is referenced within a transaction, no other transaction can perform DDL such as DROP TABLE or ALTER TABLE until the first transaction commits. Previously, the lock was released at the end of a statement rather than the whole transaction.
Improved Performance and Scale on Win32 and Win64
If your company uses Windows by itself or in a mixed environment, you probably want to deploy MySQL databases on Windows. To make that a reality, the MySQL team has incorporated a number of Windows-specific features for speeding up and scaling up.
* Windows API calls for much of the I/O done inside MySQL (a community contribution, hat tip to Jeremiah Gowdy). * Ability to build engines and other plugins as DLLs on Windows. * Network support for auto-detecting the MAC address (a community contribution, hat tip to Chris Runyan). * Much cleanup and simplifying of threading code. Improved Availability
Semi-Synchronous Replication
This feature improves the reliability of failover, to avoid failing over to a slave that is missing some committed changes from the master. You can choose to have commits on the master node wait until at least one slave has logged the relevant events for the transaction. The "semi-synchronous" aspect is because the master does not wait for all the slaves to acknowledge, and there is a protocol to avoid the master waiting too long if the slaves fall behind. Read more about semisynchronous replication. Replication Heartbeat
In replication, the "heartbeat" is a message sent at regular intervals from a master node to the slave nodes. You can configure the heartbeat period. If the message is not received, the master knows that the slave node has failed. You can now avoid the spurious relay log rotation when the master is idle, rely on an more precise failure detection mechanism, and have an accurate estimation for seconds behind master. (This is a different feature than "Linux heartbeat", which is a similar health-checking system for cluster nodes.) To use this feature, you issue commands like:
CHANGE MASTER SET master_heartbeat_period= milliseconds; SHOW STATUS like 'slave_heartbeat period' SHOW STATUS like 'slave_received_heartbeats'
Improved Usability SIGNAL/RESIGNAL
The SIGNAL and RESIGNAL statements allow you to implement familiar exception-handling logic in your stored procedures, stored functions, triggers, events, and database applications that call those things. SIGNAL passes execution back to an error handler, like THROW or RAISE statements in other languages. You can encode the error number, SQLSTATE value, and a message in a consistent way that can be interpreted by an error handler in the calling program. RESIGNAL lets you propagate the exception after doing some amount of error handling and cleanup yourself. With RESIGNAL, you can pass along the original error information or modify it. Read more about SIGNAL/RESIGNAL. More Partitioning Options
With the new RANGE COLUMNS and LIST COLUMNS clauses of the CREATE TABLE statement, partitioning is now more flexible and also can optimize queries better. Instead of expressions, you specify the names of one or more columns. Both of these clauses let you partition based on DATE, DATETIME, or string values (such as CHAR or VARCHAR). Partition pruning can optimize queries on tables that use RANGE COLUMNS or LIST COLUMMS partitioning, and WHERE conditions that compare different columns and constants, such as
a = 10 AND b > 5
or
a < "2005-11-25" AND b = 10 AND c = 50
Performance Schema
The Performance Schema feature involves an optional schema, named performance_schema, with tables that you can query to see intimate details of low-level MySQL performance. You can get information about performance right at that moment, or various amounts of historical performance data. You can clear the data to reset the figures, filter and format the data using WHERE clauses, and generally interact with it using all sorts of SQL goodness. Performance Schema data now also includes details about the InnoDB storage engine. Read more about Performance Schema.
