Although the most common use of the MySQL database server replication facility is master-slave replication, master-master replication (where each is both a master, and a slave to the other) can be extremely useful in maintaining a hot-spare database server for emergency use.

Improving the performance of a system using the MySQL database server is not just about indexes and buffers – getting the most out of the system requires a wider view. We’ll start by looking at what needs to be considered when a system starts needing work to maintain or improve performance.

Without wishing to sound pedantic, they’re indexes build upon a prefix of the column’s data.

‘When deciding whether to create a new index, or to split a table, it”s useful to work out the cardinality of the data to see whether it”s worth doing. It”s possible to calculate the cardinality of the data in a column before creating an index.

The knee-jerk reaction to poor query performance is ”Create an index!”, but this is not always guaranteed to improve query performance, and in some cases can even damage it.

Cardinality refers to how many unique values a particular column contains with respect to the number of rows in that table.