{
  system ? builtins.currentSystem,
  nixpkgs ? builtins.fetchTarball {
    url = "https://github.com/NixOS/nixpkgs/archive/26.05.tar.gz";
    sha256 = "sha256:0am8xx09fx5yf2p0wb001v0jx1g5hrfb76h4r37xph378jgk7pcr";
  }
}:
let
  # use the first version in the list for each component
  getMajorVersion = builtins.getEnv "PPQ_POSTGRES_VERSION";

  overlay = self: super: {
    # NOTE: Overlaying `postgresql` is not possible anymore, leads to
    # error: infinite recursion encountered
    #
    # TODO: Make extension packages more easily configurable.  For now
    # just load whatever we've ever needed.
    ppq_postgresql = super."postgresql_${getMajorVersion}".withPackages (p: [
      p.postgis
    ]);
  };

  pkgs =
  import nixpkgs {
    inherit system;
    overlays = [
      overlay
    ];
  };
in
pkgs