Logging all MySQL queries into the Slow Log

A country road with signs asking to slow down.
Is this road’s traffic logged somehow?

MySQL optionally logs slow queries into the Slow Query Log – or just Slow Log, as friends call it. However, there are good reasons to log all the queries, not just some of them. This article shows how to log all available information into the Slow Log.

Why can’t I only log the slowest queries?

There are several reasons to log all queries. This list is not exhaustive:

  • If you have a locking query that takes 0.5 seconds and runs 10 per second, most probably it’s much more relevant than a query that takes 1 second and is executed once per second.
  • Threshold are not smart. Put a 5 seconds threshold, and you will never know that a very frequent query takes 4.9 seconds.
  • Suppose you only log queries that take 5 or more seconds. If a query takes less than 1 second more of the times and ore than 5 seconds in rare cases (which typically depends on data distribution) you will lose this information.
  • If you don’t log relatively fast queries, you cannot analyse your workload and see, for example, that many locking statements insist on the same table.
  • You want to see and remove useless queries.
  • For some queries it is perfectly ok to take a long time (one-time queries, cached queries, analytics…). This makes any time threshold not much significant.

How to apply the changes

Below you can find the variables to change, as they should be written in the configuration file (most probably /etc/mysql/my.cnf or /etc/my.cnf). Note that changes in the configuration file will take effect on MySQL restart.

You avoid restarting MySQL, but I recommend to make the changes to the configuration file anyway, in case it crashes.

To change variables values at runtime:

SET GLOBAL variable_name := 1;
SET GLOBAL variable_name := 'value'; # string values

This only takes effect for new connections. You are not logging all queries until all existing connections are closed and new ones are established.

Setting the Slow Log

Here you can find the correct setting to use to log all your queries, depending on which MySQL flavour you use.

MySQL

8.0

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
log_throttle_queries_not_using_indexes = 0
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

# information in the slow log
log_slow_extra = 1

5.7

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
log_throttle_queries_not_using_indexes = 0
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

5.6

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
log_throttle_queries_not_using_indexes = 0
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

Percona Server

8.0

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
log_throttle_queries_not_using_indexes = 0
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1
slow_query_log_use_global_control = all

# information in the slow log
log_slow_extra = 1
log_slow_verbosity = full

5.7

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
log_throttle_queries_not_using_indexes = 0
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1
slow_query_log_use_global_control = all

5.6

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
log_throttle_queries_not_using_indexes = 0
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1
slow_query_log_use_global_control = all

MariaDB

10.5

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

# information in the slow log
log_slow_verbosity = query_plan,innodb,explain

10.4

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

# information in the slow log
log_slow_verbosity = query_plan,innodb,explain

10.3

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

# information in the slow log
log_slow_verbosity = query_plan,innodb,explain

10.2

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

# information in the slow log
log_slow_verbosity = query_plan,innodb,explain

10.1

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1
log_slow_admin_statements = 1
log_slow_slave_statements = 1

# information in the slow log
log_slow_verbosity = query_plan,innodb,explain

10.0

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1

# information in the slow log
log_slow_verbosity = query_plan,innodb,explain

5.5 and previous versions

# log queries into a log.slow file
slow_query_log = 1
log_output = FILE
slow_query_log_file = log.slow

# log all queries
long_query_time = 0
min_examined_row_limit = 0
log_queries_not_using_indexes = 1

Is the Slow Log slow?

In most cases, the slow log performance impact is minimum. It may sound scary to think that MySQL will write something into a file for every single statement; but not if you know the amount of IO that is required for every single statement (undo log, redo log, binlog, potentially reads from tablespaces) and InnoDB background threads. And the slow log is just a sequential write-only file.

That said, if your IO is nearly saturated, Percona Server and MariaDB allows to throttle the slow log. Which means, multiple queries will be written to the file together, to reduce the IO operations. Take a look at log_slow_rate_limit. Note however that, if you do so, some queries will not be logged.

EDIT: Thanks Peter Zaitsev for reporting to me that log_slow_rate_limit was first implemented and is also supported in Percona Server. He wasn’t able to write a comment because of a problem in this website, which I am investigating. Please let me know if you are also experiencing any problems.

See also

Related courses

Reference

Conclusions

We discussed why you should log all your queries and all available information about them. We saw exactly how to do it, depending on your MySQL or Percona Server or MariaDB version.

This is something that I check at the beginning of my MySQL Health Checks. This is because I include recommendations so optimise the most impacting queries, so I need to be sure to have complete query statistics.

If you spot any mistake, or if you have more ideas on how to get the most from the Slow Log, please drop a comment below.

Remember: your comments are valuable and welcome!

Toodle pip,
Federico

Photo credit

Leave a Reply

Your email address will not be published. Required fields are marked *