Pass postgres parameter into Kubernetes deployment

In my case, the @Rico answer didn't help me out of the box because I don't use postgres with a persistent storage mount, which means there is no /var/lib/postgresql/data folder and pre-existed database (so both proposed options have failed in my case).

To successfully apply postgres settings, I used only args (without command section).

In that case, k8s will pass these args to the default entrypoint defined in the docker image (docs), and as for postgres entrypoint, it is made so that any options passed to the docker command will be passed along to the postgres server daemon (look section Database Configuration at: https://hub.docker.com/_/postgres)

apiVersion: v1
kind: Pod
metadata:
  name: postgres
spec:
  containers:
    - image: postgres:9.6.8
      name: postgres
      args: ["-c", "shared_buffers=256MB", "-c", "max_connections=207"]

To check that the settings applied:

$ kubectl exec -it postgres -- bash
root@postgres:/# su postgres
$ psql -c 'show max_connections;'
 max_connections
-----------------
 207
(1 row)

You didn't share your Pod/Deployment definition, but I believe you want to set shared_buffers from the command line of the actual container (not the init container) in your Pod definition. Something like this if you are using a deployment:

apiVersion: v1
kind: Deployment
metadata:
  name: postgres
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
        - name: postgres
          image: postgres:12.2
          imagePullPolicy: "IfNotPresent"
          command: ["postgres"] # <-- add this
          args: ["-D", "-c", "shared_buffers=128MB"] # <-- add this
          ports:
            - containerPort: 5432
          securityContext:
            runAsUser: 1000
            runAsGroup: 1000
            fsGroup: 1000
          volumeMounts:
            - mountPath: /var/lib/postgresql/data
              name: postgredb
            - name: postgresql-config-volume # <-- use if you are using a ConfigMap (see below)
              mountPath: /var/lib/postgres/data/postgresql.conf
      volumes:
        - name: postgredb
          persistentVolumeClaim:
            claimName: postgres-pv-claim # <-- note: you need to have this already predefined
        - name: postgresql-config-volume # <-- use if you are using a ConfigMap (see below)
          configMap:
            name: postgresql-config

Notice that if you are using a ConfigMap you can also do this (note that you may want to add more configuration options besides shared_buffers):

apiVersion: v1
kind: ConfigMap
metadata:
  name: postgresql-config
data:
  postgresql.conf: |
    shared_buffers=256MB