MySQL Troubleshooting

Descriptions of Variables

Now that you are familiar with risk-free tuning methods for server variables, we are ready to discuss a few important ones. This is not a complete guide, but a starting point for further research.

You can read the rest of this section in any order: go through it from beginning to end, check the particular topics of most interest to you at the moment, or even skip it and use it as a reference when you encounter a problem. I don’t describe every option, but concentrate on those that I’ve found are often used improperly or require an expanded understanding.

Note

At first, I doubted whether I should devote a section of this book to individual variables because each one is fully described in other sources. But the MySQL Reference Manual does not describe them specifically from a troubleshooting approach, so I decided to offer this short overview.

Options That Affect Server and Client Behavior

This section discusses general server options as well as options related to replication, connections, and storage engines. In the latter set, I’ll cover only the MyISAM and InnoDB storage engines.

Server-related options

These affect all connections and statements.

Limits and max_* variables

You already saw how max_allowed_packet affects an application earlier in Options That Limit Hardware Resources. Other options restrict the size of result sets. Examples include group_concat_max_len, which limits the number of bytes that the GROUP_CONCAT function can return. Just compare:

mysql> ...