mysql: related optimization

1. How do you view sql statement execution efficiency?

Use explain the function:

1	explain select * from user ;

Or turn on slow query
whether to open

1	show variables like '% or%' ;

log_slow_queries = off does not turn on slow query
long_query_time is slow query time, sql statement than this time will generate a log? In general more than three seconds slower even

Original link large column  https://www.dazhuanlan.com/2019/08/16/5d5608f12197b/