With the migration to MySQL 5.5 there are some new features that have been introduced, where some aren’t compatible with the previous MySQL 5.1 server that was running on our servers.
With MySQL 5.5 InnoDB has become default the storage engine instead of MyISAM storage engine.
Improved performance is due to the optimizations in MySQL to better utilize multi core CPUs alongside with improving InnoDB locking and memory management. Since all of our servers have multiple cores available to services running on them, it is important that they can fully utilize them and provide maximum efficiency.
On the numeric side, the server includes a new a library for conversions between strings and numbers, dtoa.
This library provides the basis for an improved conversion between string or DECIMAL values and approximate-value (FLOAT or DOUBLE) numbers. Also, all numeric operators and functions on integer, floating-point and DECIMAL values throw an out of range error (ER_DATA_OUT_OF_RANGE) rather than returning an incorrect value or NULL.
If an application relies on previous numeric results, it may have to be adjusted to the new precision or behavior.
The Unicode implementation has been extended to provide support for supplementary characters that lie outside the Basic Multilingual Plane (BMP), introducing the utf16, utf32 and utf8mb4 charsets.
If you are considering upgrading from utf8 to utf8mb4 to take advantage of the supplementary characters, you may have to adjust the size of the fields and indexes in the future. See http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-upgrading.html.
Upgrading to utf8mb4 will not take place unless you explicitly change the charset, i.e. with a ALTER TABLE… statement
The stopword file is loaded and searched using latin1 if the character_set_server is ucs2, utf16, or utf32. If any table was created with FULLTEXT indexes while the server character set was ucs2, utf16, or utf32, it should be repaired using this statement REPAIR TABLE tbl_name QUICK;
MySQL 5.5 introduces a number of deprecated features that would be removed in future MySQL releases. Certain features are provided with alternatives that can be used, other features would be fully removed.
For a in depth review of the changes introduced in MySQL 5.5 please visit: http://dev.mysql.com/doc/refman/5.5/en/mysql-nutshell.html