PostgreSQL's logs - a developer's best friend

March 2018 · 3 minute read

Introduction

A few years back, when I was an intern, the CTO asked me if I checked how many queries were executed on the page, turns out there were over a hundred…
I realized that I was missing some basics :

  • Like a lot of web developers I didn’t think of looking into my logs to know what my ORM was doing
  • Once I knew, I didn’t understand why some queries were slow

After that I became a bit obsessed with performances and that’s the story of why I submitted a talk at the django con 2017, and then at the pgdayParis 2018.

The conference at pgdayParis lasting 50 minutes, I had a bit more time to get into details but if you’re using Django’s ORM, maybe the Djangocon Europe version is more interesting. I’ll let you decide ;)

And here are the slides

I am currently writting a series of articles on the subjects covered during this talk. This first one is about logs, why and how we, developers, should always look into our logs !

Why should we be careful about ORMs

Alt text
Most developers tend to work with an ORM for many reasons that we are not going to develop here, but the question is: what’s wrong with them?
Why are DBAs always saying they are evil and anyone using them should burn in hell (hell being somewhere without beer) ?

  • The ORM executes queries that you might not expect
  • Your queries might not be optimised and you won’t know about it

I’m going to give the example of the loops to show you how ugly it can get if you’re not careful…

owls = Owl.objects.filter(employer_name=‘Hogwarts’)
  for owl in owls:
        print(owl.job)

With this piece of django code you would end up in your logs with those queries:

SELECT id, name, employer_name, favourite_food, job_id, feather_color FROM owl WHERE employer_name = 'Hogwarts'
SELECT id, name FROM job WHERE id = 1
SELECT id, name FROM job WHERE id = 1
SELECT id, name FROM job WHERE id = 1
SELECT id, name FROM job WHERE id = 1
SELECT id, name FROM job WHERE id = 1
SELECT id, name FROM job WHERE id = 1
SELECT id, name FROM job WHERE id = 1
SELECT id, name FROM job WHERE id = 1
…

So it’s pretty obvious that ORMs have to be handled with a lot of care if you don’t want to have a slow website and an angry DBA.

So where are my logs ?

If you’re using an ORM and have never looked for your logs, let me show you how to find them.

First go in your psql:

 $ psql -U user -d your_database_name

Then you will find the path of your log files.


owl_conference=# show data_directory ;
     data_directory
-------------------------
 /usr/local/var/postgres

owl_conference=# show log_directory ;
 log_directory
---------------
 pg_log

owl_conference=# show log_filename ;
      log_filename
-------------------------
 postgresql-%Y-%m-%d.log

So in this case you would find your logs in

/usr/local/var/postgres/pg_log/postgresql-2018-03-23.log

If you’re a developer, working localy, in order to log all queries, you need to change your postgresql.conf and put

log_statement = 'all'
logging_collector = on
log_min_duration_statement = 0

If you can’t find your configuration file, you can run in psql:

owl_conference=# SHOW config_file;
config_file
-----------------------------------------
 /usr/local/var/postgres/postgresql.conf
(1 row)

Conclusion

So here we are, you have your logs and you know why it’s so important to look into them ! The next step would be having something to help you find slow queries with pg_stat_statements :)