ERROR 1238 (HY000): Variable 'log_slow_queries' is a read only variable

mysql> set global log_slow_queries = ON;              

ERROR 1238 (HY000): Variable 'log_slow_queries' is a read only variable

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One way around this is to setup the machine with the Slow Query Log 

enabled but to use a very large value of --long-query-time to 

essentially ignore every query. Then, when you want to capture slow 

queries, you reset --long-query-time to a reasonable value. 

Unfortunately, this requires a restart to initialize. After that you can 

adjust the --long-query-time to throttle the contents of the log.

mysql5.0 버전까지는 재시작없이는 설정이 안된단다. my.cnf에 등록하고 db 재시작해야한다. 5.1부터는 재시작없이 설정가능