August 21, 2014

How MySQL ‘queries’ and ‘questions’ are measured

MySQL has status variables “questions” and “queries” which are rather close but also a bit different, making it confusing for many people. The manual describing it might not be very easy to understand:

In a nutshell if you’re not using prepared statements the big difference between those is what “Questions” would count stored procedure calls as a single statement where “Queries” will count all statements called inside of stored procedures as well.

There seems to be also more subtle difference between them:

Running on MySQL 5.6.17 having created a new connection I see…

Which tells me what “queries” is a global status variable while “questions” is a session and can be used to see how many statements were issued to the server through the current connection.

There is also a global questions variable that shows the number for a server since the start:

When it comes to global values neither “queries” nor “questions” are reset when FLUSH STATUS is called.

What the manual does not describe in detail though is: When exactly those counters are incremented? This might look irrelevant but really it is not, especially if you’re capturing those values with high resolution and using them to diagnose non trivial MySQL performance incidents.

If you would count queries when they start – when a spike in the amount of queries in the given second could be due to the spike in the traffic, however, and if you measure queries at the completion – spikes could also mean that some critical resource became available which allowed for many queries to complete. Think for example of table-level locks or row-level locks with Innodb as very common cause.

So what is the case with MySQL? MySQL increments this counter Before executing the query so you may well see a very uniform rate of questions when actually a lot of queries were started but were not completing quickly due to waiting on some resource.

To check for unusual numbers of queries running concurrently and struggling to complete in time looking at threads_running status variable is a great idea.

About Peter Zaitsev

Peter managed the High Performance Group within MySQL until 2006, when he founded Percona. Peter has a Master's Degree in Computer Science and is an expert in database kernels, computer hardware, and application scaling.

Comments

  1. Hi Peter,

    A not-that-interesting question. Is there a way to reset the global counters for the questions and the queries? Will restarting MySQL reset those counters?

  2. Fadi,

    Nope FLUSH STATUS does not resent them. This command BTW is quite confusing which resets local (connection) counters flushing them to global counters but when it also zeroes out some of the global counters but not all of them.

    As such it is much better to compute deltas in the application if you’re using it for monitoring.

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