Log query before executing it - mysql

Question

I've run into a problem where I run some query and the mysqld process starts using 100% CPU power, without ending. I want to pinpoint this query. The problem is that log/development.log contains only queries that have finished. Any idea?

Answer 1

I think you have a few options for this. The first is really taking a look at your development.log and seeing which actions are causing it. Take a look at the queries you're asking rails to run and try to pinpoint that particular query. If it's taking a large amount of time it probably means you're doing something like returning n+1 queries, missing indexes or some other performance killer.
You say that the dev log only has queries that have finished. Can't you work out what the next query to run would be?
Your other options involve starting mysqld with a log (i think the names of some of these have changed):
mysqld --log[=file_name] --log-slow-queries[=file_name]
Showing the current statement list using processlist from within mysql:
show processlist;
To prevent stuff like this happening again you could also take some time to look at a rails performance monitor like RPM from New Relic (http://www.newrelic.com/).
I hope this helps!

Answer 2

You could take a look at running/unfinished statements via the
show processlist;
command.

Answer 3

If you have assess to MySQL, consider the SQL query
SHOW PROCESSLIST
Or from the command line:
mysqladmin processlist
Alternatively, the most powerful way is to override the 'execute' method of the ActiveRecord::Base connection instance. This article shows the general approach:
http://www.misuse.org/science/2006/12/12/sql-logging-in-rails/
You put this code into application.rb:
# define SQL_LOG_FILE, SQL_LOG_MAX_LINES
connection = ActiveRecord::Base.connection
class << connection
alias :original_exec :execute
def execute(sql, *name)
# try to log sql command but ignore any errors that occur in this block
# we log before executing, in case the execution raises an error
begin
lines = if File::exists?(SQL_LOG_FILE) then IO::readlines(SQL_LOG_FILE) else [] end
log = File.new(SQL_LOG_FILE, "w+")
# keep the log to specified max lines
if lines.length > SQL_LOG_MAX_LINES
lines.slice!(0..(lines.length-SQL_LOG_MAX_LINES))
end
lines << Time.now.strftime("%x %I:%M:%S %p")+": "+sql+"n"
log.write(lines)
log.close
$sql_log = sql
rescue Exception => e
;
end
# execute original statement
original_exec(sql, *name)
end # def execute
end # class <<