Understanding MySQL Slow Query Logs

Hello all, here we are going to discuss about one of best performance problem solving method i.e. to understand the MySQL slow query logs and take important measures to ensure peak server performance.


Let’s take an example of Slow Query Log:

root@server#tail -f /var/log/mysql_slow_query.log
# Time: 2017-04-09T20:10:09.472872Z
# User@Host: db_user[db_name] @  [x.x.x.125]
# Query_time: 3.776369  Lock_time: 0.000269 Rows_sent: 0  Rows_examined: 308448
SET timestamp=1491768609;
insert into tempo_table select * from tempo_table_tmp;

Now let’s understand what each line indicates:


  • The first line shows the time the query was logged. The format is YYMMDD H:M:S. We can see the query above was logged at 2017 April 9th at 20:10 –

NOTE: This is server time, which may be different from your local time.

  • Next, we’ll see the MySQL user, database, and hostname/ip address.
  • The third line shows the total query time, the Lock time, the number of Rows “sent” or returned, and the number of rows examined during the query.
  • Below that you’ll see SET timestamp=UNIXTIME; < this is the time the query actually happened. It is good to check that you’re not looking at a problem that happened months ago if you’re trying to figure out why something is slow right now. I’ll explain how to turn that into a usable time below.
  • The final line shows the full query.


How to make Timestamp human-readable:

root@server# date -d @1491768609
Mon Apr 10 01:40:09 IST 2017

This will work on linux/unix.


I hope this helps let me know via comments if any information is not correct or missing.

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