Here’s a little trivia that you might find helpful. Suppose that I have the following in the MySQL slow query log (abbreviated for clarity):
# User@Host: root[root] @ localhost [127.0.0.1] # Time: 100919 17:58:52 # Query_time: 9.648427 Lock_time: 8.648039 select sleep(1) from t limit 1;
To get this into the slow query log, I set the long_query_time to 0 and opened two sessions. In one session I ran
LOCK TABLES t WRITE, and in the other I tried to select from that table. As you can see above, 1) LOCK TABLES contributes to the Lock_time number, and 2) the Query_time is the sum of execution time and lock time.
Now, I’ll set long_query_time = 2 and run the same test. What happens? Nothing shows up in the slow query log, because 3) the[Read more...]