Questions

Questions and Answers

23.1.1: Which version of MySQL is production-ready (GA)?

Currently, both MySQL 5.0 and MySQL 5.1 are supported for production use.

MySQL 5.0 achieved General Availability (GA) status with MySQL 5.0.15, which was released for production use on 19 October 2005.

MySQL 5.1 achieved General Availability (GA) status with MySQL 5.1.30, which was released for production use on 14 November 2008.

MySQL 5.4 was a development series. Work on this series has ceased.

Development work on MySQL 5.5 has started; currently, MySQL 5.5 is in beta status.

Development work on MySQL 6.0 has started; currently, MySQL 6.0 is in alpha status.

23.1.2: Can MySQL 5.5 do subqueries?

Yes. See Section 12.2.10, “Subquery Syntax”.

23.1.3: Can MySQL 5.5 perform multiple-table inserts, updates, and deletes?

Yes. For the syntax required to perform multiple-table updates, see Section 12.2.12, “UPDATE Syntax”; for that required to perform multiple-table deletes, see Section 12.2.2, “DELETE Syntax”.

A multiple-table insert can be accomplished using a trigger whose FOR EACH ROW clause contains multiple INSERT statements within a BEGIN ... END block. See Section 18.3, “Using Triggers”.

23.1.4: Does MySQL 5.5 have a Query Cache? Does it work on Server, Instance or Database?

Yes. The query cache operates on the server level, caching complete result sets matched with the original query string. If an exactly identical query is made (which often happens, particularly in web applications), no parsing or execution is necessary; the result is sent directly from the cache. Various tuning options are available. See Section 7.5.5, “The MySQL Query Cache”.

23.1.5: Does MySQL 5.5 have Sequences?

No. However, MySQL has an AUTO_INCREMENT system, which in MySQL 5.5 can also handle inserts in a multi-master replication setup. With the auto_increment_increment and auto_increment_offset system variables, you can set each server to generate auto-increment values that don't conflict with other servers. The auto_increment_increment value should be greater than the number of servers, and each server should have a unique offset.

23.1.6: Does MySQL 5.5 have a NOW() function with fractions of seconds?

No. This is on the MySQL roadmap as a “rolling feature”. This means that it is not a flagship feature, but will be implemented, development time permitting. Specific customer demand may change this scheduling.

However, MySQL does parse time strings with a fractional component. See Section 10.3.2, “The TIME Type”.

23.1.7: Does MySQL 5.5 work with multi-core processors?

Yes. MySQL is fully multi-threaded, and will make use of multiple CPUs, provided that the operating system supports them.

23.1.8: Why do I see multiple processes for mysqld?

When using LinuxThreads, you should see a minimum of three mysqld processes running. These are in fact threads. There is one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

23.1.9: Is there a hot backup tool for MyISAM like InnoDB Hot Backup?

This is currently under development for a future MySQL release.

23.1.10: Have there been there any improvements in error reporting when foreign keys fail? Does MySQL now report which column and reference failed?

The foreign key support in InnoDB has seen improvements in each major version of MySQL. Foreign key support generic to all storage engines is scheduled for MySQL 6.x; this should resolve any inadequacies in the current storage engine specific implementation.

23.1.11: Can MySQL 5.5 perform ACID transactions?

Yes. All current MySQL versions support transactions. The InnoDB storage engine offers full ACID transactions with row-level locking, multi-versioning, nonlocking repeatable reads, and all four SQL standard isolation levels.

The NDB storage engine supports the READ COMMITTED transaction isolation level only.