{
  config,
  pkgs,
  ...
}: {
  environment.systemPackages = let
    # script from:
    # https://nixos.org/manual/nixos/stable/index.html#module-postgresql
    # 1) bump toVersion; rebuild
    # 2) run this script
    # 3) bump postgres; rebuild
    # 4) run analyze_new_cluster.sh; delete_old_cluster.sh from the database dir.
    toVersion = "14"; # Bump this when there is a new version.
    new_package = "postgresql_${toVersion}";
  in [
    (pkgs.writeScriptBin "upgrade-pg-cluster" ''
      set -eux

      systemctl stop postgresql

      export NEWDATA="/var/lib/postgresql/${toVersion}"

      export NEWBIN="${pkgs.${new_package}}/bin"

      export OLDDATA="${config.services.postgresql.dataDir}"
      export OLDBIN="${config.services.postgresql.package}/bin"

      install -d -m 0700 -o postgres -g postgres "$NEWDATA"
      cd "$NEWDATA"
      sudo -u postgres $NEWBIN/initdb -D "$NEWDATA"

      sudo -u postgres $NEWBIN/pg_upgrade \
        --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
        --old-bindir $OLDBIN --new-bindir $NEWBIN \
        "$@"
    '')
  ];
}