postgres - c <parameter> = <value> not working for postgres 11 in docker

Question

I'm running a Postgres inside a docker. I want to change the default config of Postgres so I'm running :

docker container run -d postgres -c max_connections=200 -c shared_buffers = 1GB -c effective_cache_size=3GB -c maintenance_work_mem=256MB -c checkpoint_completion_target=0.7 -c wal_buffers=16MB 

But when I'm connecting to Postgres running:

 docker exec -it container_name psql

And then the result of :

SHOW max_connections;

is

 max_connections
-----------------
 100
(1 row)

And it's not just max_connections. None of the parameters are changed. And I don't know what is the problem with what I'm doing?

Update: I already have a running docker container and I want it to apply these parameters to it without needing to restart it.

Answer 1

You cannot put spaces around the equals sign when using the "-c" option. So you need something like postgres -c max_connections=200

When I try it with the spaces, it refused to start at all. So I don't know what you are connecting to in order to SHOW max_connections;. Maybe you already had a server running before you tried to start with the invalid syntax?

Answer 2

When running Postgresql with Docker, there are many ways to change configuration when starting a container.

It is explained in the Docker image official documentation at this paragraph : Database Configuration.

In our case, there is an example :

$ docker run -d --name some-postgres -e POSTGRES_PASSWORD=mysecretpassword postgres -c shared_buffers=256MB -c max_connections=200

So let's try this with Postgres 14 :

$ docker run -d --name pg14 -e POSTGRES_PASSWORD=mysecretpassword postgres:14 -c shared_buffers=256MB -c max_connections=200

Check it with psql :

$ docker exec -it pg14 psql -U postgres

If we execute SHOW max_connections;, 200 is displayed, so it worked 👍.

⚠️ Setting this parameter on the command line will indeed override the configuration in postgresql.conf.

We can check the configuration file used by the container with :

$ docker exec pg14 cat /var/lib/postgresql/data/postgresql.conf

It contains :

max_connections = 100                   # (change requires restart)

Even if the max_connections parameter has been successfully changed on the command line, in the logs we still see the default value (from the config file I guess) :

$ docker logs pg14  | grep max_connections
...
selecting default max_connections ... 100
...