MySQL Troubleshooting

How to Check Whether Changes Had an Effect

There are status variables that show the current server status. As opposed to configuration variables, these do not affect server behavior, but instead contain information about what is happening in the mysqld process. Status variables are read-only, i.e., it is the MySQL server that changes them, not the user. They show such things as how many queries were executed and of what kind, network traffic, how indexes are used (you can find an example in Queries That Modify Data), buffer usage, how many tables are open, how many temporary tables were created, and a lot of other useful information. I won’t describe each of them here, but will note which status variable to watch while covering the variables themselves later in this chapter, as I did in Queries That Modify Data.

In the context of altering variables, the status variables are useful mostly in Performance-Related Options. We will also discuss how to get information about what is going on in your MySQL configuration in Chapter 6.

Like other variables, status variables can apply to both individual sessions and all sessions (global). Session variables show the status for the current session, whereas global variables show the status since the server was started or since the last FLUSH STATUS command was executed.

Note

Some variables are not independent. For example, variables that control the query cache change nothing when query_cache_size is set to zero. When tuning such options, always ...