Stay Curious!

How LOCK TABLES interacts with the MySQL slow query log

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 time spent waiting for table locks doesn’t count towards the slow query time threshold.

A final note: it was rumored that the LOCK TABLES query itself is somehow a special-case that is never logged to the slow query log. However, this is not true; if long_query_time is set to zero, the LOCK TABLES query will appear in the log.

Posted on Sun, Sep 19, 2010. Approximately 200 Words.

Databases