pg_stat_statements - finding ugly queries

March 2018 · 2 minute read

Introduction

As a reminder, if you ended up on this page by looking up on bing pg_stat_statements, first I’m impressed, you get results with bing ? But also, this article is based on a talk that I did at the pgdayParis 2018 and here are the slides

What is pg_stat_statements ?

pg_stat_statement is a postgreSQL extension, once you enable it, it tracks statistics on the queries executed by a server. It will help you find slow queries.
Alt text

You can use it in your local environment but also on production, if you are afraid of the performances loss, here is an article on the subject.

How can I activate it ?

First go in your psql:

 $ psql -U owl -d owl_conference

And create the extension. Only superuser or database owner can create extensions. So you might need to check the privileges of the user you are connecting with.

CREATE EXTENSION pg_stat_statements;

You then have to change your postgresql.conf file and restart. You can’t only reload your conf file because pg_stat_statements needs to be added to the shared_preload_libraries. So here is the configuration.

shared_preload_libraries = 'pg_stat_statements'
pg_stat_statements.max = 10000
pg_stat_statements.track = all

You can change the pg_stat_statements.max if you want to track more queries or are afraid of the size of the table.

And now what?

Well, now you can simply find statistics about the queries tracked by running

SELECT total_time, min_time, max_time, mean_time, calls, query
FROM pg_stat_statements
ORDER BY mean_time DESC
LIMIT 100;

You will get a set of results looking like:

-[ RECORD 1 ]---------------------------------------------------------
total_time | 643.624
min_time   | 54.424
max_time   | 531.417
mean_time  | 214.541333333333
calls      | 3
query      | SELECT COUNT(*) FROM letters;

So from that you understand:

  • The query has been executed 3 times (calls)
  • The execution time was between 54 and 531 ms (min_time, max_time)
  • The total_time is the sum of the time of each execution
  • The mean_time is total_time/calls


Conclusion

Well that’s it ! But now that you know which queries are slow, you probably want to understand what’s wrong. To do that, I encourage you to use EXPLAIN, if you want to read about it, I am currently writing articles on EXPLAIN, so be patient, they should be available in the next few days :)