Allow setting the default MotherDuck database at init

[mjkanji]

What happens?

Problem

In the pgduckdb/pgduckdb Docker image, setting MOTHERDUCK_TOKEN (or motherduck_token) implicitly enables MotherDuck during initialization.

This happens because the image init script unconditionally runs CALL duckdb.enable_motherduck(<token>) when it detects the token env var:

• docker/init.d/0002-enable-md-pg_duckdb.sql:
  • reads MOTHERDUCK_TOKEN / motherduck_token
  • calls duckdb.enable_motherduck(:token) if present

Why this is an issue

duckdb.enable_motherduck(token, default_database) only applies default_database when the motherduck foreign server is created. If the Docker init script has already created the server (without default_database), users cannot set the default database at startup.

This effectively forces users into the “drop/recreate server” workflow to change the default database, as documented here:
https://github.com/duckdb/pg_duckdb/blob/main/docs/motherduck.md#changing-the-default-database

In practice this makes the container’s “just set MOTHERDUCK_TOKEN” path hard to use for real deployments where the default DB needs to be controlled declaratively at boot time.

Requested change

Please consider one of the following:

1. Add an env var for the default database (e.g. MOTHERDUCK_DEFAULT_DATABASE) and update the init script to call:
   • CALL duckdb.enable_motherduck(:token, :'default_db'); when provided, otherwise keep current behavior.

or

2. Disable automatic enabling by default, and require an explicit opt-in env var (e.g. PGDUCKDB_ENABLE_MOTHERDUCK=1) before the init script runs duckdb.enable_motherduck(...).

Either option would preserve a good “it just works” experience while making it possible to configure default_database cleanly at container start.

Expected outcome

• Users can set both MOTHERDUCK_TOKEN and the default MotherDuck database non-interactively at init time, without needing to drop/recreate the server afterward.

To Reproduce

docker run -d -e POSTGRES_PASSWORD=duckdb -e MOTHERDUCK_TOKEN pgduckdb/pgduckdb:18-v1.1.1

OS:

Linux

pg_duckdb Version (if built from source use commit hash):

1.1.1

Postgres Version (if built from source use commit hash):

18

Hardware:

No response

Full Name:

Muhammad Jarir Kanji

Affiliation:

Indus Data

What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have tested with a stable release

Did you include all relevant data sets for reproducing the issue?

No - Other reason (please specify in the issue body)

Did you include all code required to reproduce the issue?

• Yes, I have

Did you include all relevant configuration (e.g., CPU architecture, Linux distribution) to reproduce the issue?

• Yes, I have