Database inside the DSN is never excluded

What did you do?

Try to exclude the "master" database on custom queries with auto-discovery enabled (I don't know if it's the right term, but it is the database provided with the DSN).

What did you expect to see?

Even if I exclude the database provided inside the DSN, the exporter execute queries on it

# with DATA_SOURCE_NAME: "postgresql://postgres:password@postgres:5432/postgres?sslmode=disable" and PG_EXPORTER_EXCLUDE_DATABASES: 'postgres'
curl --silent localhost:9187/metrics | grep pg_test | grep -v '#'
pg_test_id{database="db1",server="localhost:5432"} 1
pg_test_id{database="db2",server="localhost:5432"} 1
pg_test_id{database="db3",server="localhost:5432"} 1
pg_test_id{database="db4",server="localhost:5432"} 1
pg_test_id{database="postgres",server="localhost:5432"} 1

What did you see instead? Under which circumstances?

I don't want to see the excluded database

# with DATA_SOURCE_NAME: "postgresql://postgres:password@postgres:5432/postgres?sslmode=disable" and PG_EXPORTER_EXCLUDE_DATABASES: 'postgres'
curl --silent localhost:9187/metrics | grep pg_test | grep -v '#'
pg_test_id{database="db1",server="localhost:5432"} 1
pg_test_id{database="db2",server="localhost:5432"} 1
pg_test_id{database="db3",server="localhost:5432"} 1
pg_test_id{database="db4",server="localhost:5432"} 1

Environment

• System information:

Linux 6.2.9-200.fc37.x86_64 x86_64

• postgres_exporter version:

postgres_exporter, version 0.12.0 (branch: HEAD, revision: f9a1edbbc6d629663aa658378e1d989293e6e488)
  build user:       root@463311dfdfb6
  build date:       20230321-23:52:03
  go version:       go1.20.2
  platform:         linux/amd64
  tags:             netgo static_build

• postgres_exporter flags:

(everything is configured using environment)

DATA_SOURCE_NAME: "postgresql://postgres:password@postgres:5432/postgres?sslmode=disable"
PG_EXPORTER_AUTO_DISCOVER_DATABASES: 'true'
PG_EXPORTER_DISABLE_DEFAULT_METRICS: 'true'
PG_EXPORTER_DISABLE_SETTINGS_METRICS: 'true'
PG_EXPORTER_EXCLUDE_DATABASES: 'postgres'
PG_EXPORTER_EXTEND_QUERY_PATH: /queries.yml

• PostgreSQL version:

psql (PostgreSQL) 15.2 (Debian 15.2-1.pgdg110+1)

• Logs:

Nothing relevant about logs

Personal investigation
I made some investigation to find out if I did something wrong, and here is what I found

• Files used

docker-compose.yml:

version: '3.5'

services:
  postgres:
    image: postgres
    environment:
      POSTGRES_PASSWORD: password
    volumes:
      - postgres:/data/postgres
    ports:
      - "5432:5432"
    restart: unless-stopped

  postgres-exporter:
    image: quay.io/prometheuscommunity/postgres-exporter
    environment:
      DATA_SOURCE_NAME: "postgresql://postgres:password@postgres:5432/postgres?sslmode=disable"
      PG_EXPORTER_AUTO_DISCOVER_DATABASES: 'true'
      PG_EXPORTER_DISABLE_DEFAULT_METRICS: 'true'
      PG_EXPORTER_DISABLE_SETTINGS_METRICS: 'true'
      PG_EXPORTER_EXCLUDE_DATABASES: 'postgres'
      PG_EXPORTER_EXTEND_QUERY_PATH: /queries.yml
    volumes:
      - ./queries.yml:/queries.yml
    command:
      - --log.level=debug
    ports:
      - "9187:9187"
    restart: unless-stopped
    user: root

volumes:
    postgres:

queries.yaml:

pg_test:
  query: "SELECT 1 as id, current_database() as database;"
  metrics:
    - database:
        usage: "LABEL"
        description: "Database name"
    - id:
        usage: "GAUGE"
        description: "Always 1"

• What I found ?

The default DSN is always added to the list of DSN and never filtered: https://github.com/prometheus-community/postgres_exporter/blob/master/cmd/postgres_exporter/datasource.go#L57 ... it can explain why the given database is never excluded.
My suggestion would be to delete this line, but I don't know why it exists and if there is a good reason for it.

NOTE: this can be related with https://github.com/prometheus-community/postgres_exporter/issues/735 and probably other issues like https://github.com/prometheus-community/postgres_exporter/issues/750, https://github.com/prometheus-community/postgres_exporter/issues/443

Start in cmd/postgres_exporter/datasource.go around the default DSN addition cited in the issue, then trace how discovered databases are filtered by PG_EXPORTER_EXCLUDE_DATABASES. Reproduce the configuration with auto-discovery and the custom query, and confirm that the excluded DSN database no longer produces metrics; add or update coverage if the repository provides a relevant test location.