1.6.1. What's New in MySQL 5.0

The following features are implemented in MySQL 5.0.

• BIT Data Type: Can be used to store numbers in binary notation.
• Cursors: Elementary support for server-side cursors.
• Information Schema: The introduction of the INFORMATION_SCHEMA database in MySQL 5.0 provided a standards-compliant means for accessing the MySQL Server's metadata; that is, data about the databases (schemas) on the server and the objects which they contain.
• Instance Manager: Can be used to start and stop the MySQL Server, even from a remote host.
• Precision Math: MySQL 5.0 introduced stricter criteria for acceptance or rejection of data, and implemented a new library for fixed-point arithmetic. These contributed to a much higher degree of accuracy for mathematical operations and greater control over invalid values.
• Storage Engines: Storage engines added in MySQL 5.0 include ARCHIVE and FEDERATED.
• Stored Routines: Support for named stored procedures and stored functions was implemented in MySQL 5.0.
• Strict Mode and Standard Error Handling: MySQL 5.0 added a strict mode where by it follows standard SQL in a number of ways in which it did not previously. Support for standard SQLSTATE error messages was also implemented.
• Triggers: MySQL 5.0 added limited support for triggers.
• VARCHAR Data Type: The maximum effective length of a VARCHAR column was increased to 65,535 bytes, and stripping of trailing whitespace was eliminated. (The actual maximum length of a VARCHAR is determined by the maximum row size and the character set you use. The maximum effective column length is subject to a row size of 65,532 bytes.)
• Views: MySQL 5.0 added support for named, updatable views.
• XA Transactions:
• Performance enhancements: A number of improvements were made in MySQL 5.0 to improve the speed of certain types of queries and in the handling of certain types. These include:

1. MySQL 5.0 introduces a new “greedy” optimizer which can greatly reduce the time required to arrive at a query execution plan. This is particularly noticeable where several tables are to be joined and no good join keys can otherwise be found. Without the greedy optimizer, the complexity of the search for an execution plan is calculated as N!, where N is the number of tables to be joined. The greedy optimizer reduces this to N!/(D-1)!, where D is the depth of the search. Although the greedy optimizer does not guarantee the best possible of all execution plans (this is currently being worked on), it can reduce the time spent arriving at an execution plan for a join involving a great many tables — 30, 40, or more — by a factor of as much as 1,000. This should eliminate most if not all situations where users thought that the optimizer had hung when trying to perform joins across many tables.
2. Use of the Index Merge method to obtain better optimization of AND and OR relations over different keys. (Previously, these were optimized only where both relations in the WHERE clause involved the same key.) This also applies to other one-to-one comparison operators (>, <, and so on), including = and the IN operator. This means that MySQL can use multiple indexes in retrieving results for conditions such as WHERE key1 > 4 OR key2 <> and even combinations of conditions such as WHERE (key1 > 4 OR key2 <>= 10 OR key4 = 1).

3. A new equality detector finds and optimizes “hidden” equalities in joins. For example, a WHERE clause such as

   t1.c1=t2.c2 AND t2.c2=t3.c3 AND t1.c1 < 5
   

   implies these other conditions

   t1.c1=t3.c3 AND t2.c2 < 5 AND t3.c3 < 5
   

   These optimizations can be applied with any combination of AND and OR operators.
4. Optimization of NOT IN and NOT BETWEEN relations, reducing or eliminating table scans for queries making use of them by mean of range analysis. The performance of MySQL with regard to these relations now matches its performance with regard to IN and BETWEEN.
5. The VARCHAR data type as implemented in MySQL 5.0 is more efficient than in previous versions, due to the elimination of the old (and nonstandard) removal of trailing spaces during retrieval.
6. The addition of a true BIT column type; this type is much more efficient for storage and retrieval of Boolean values than the workarounds required in MySQL in versions previous to 5.0.
7. Performance Improvements in the InnoDB Storage Engine:

> New compact storage format which can save up to 20% of the disk space required in previous MySQL/InnoDB versions.
> Faster recovery from a failed or aborted ALTER TABLE.
> Faster implementation of TRUNCATE.
8. Performance Improvements in the NDBCluster Storage Engine:
> Faster handling of queries that use IN and BETWEEN.
> Condition pushdown: In cases involving the comparison of an unindexed column with a constant, this condition is “pushed down” to the cluster where it is evaluated in all partitions simultaneously, eliminating the need to send non-matching records over the network. This can make such queries 10 to 100 times faster than in MySQL 4.1 Cluster.