{
  pkgs,
  lib,
  config,
  ...
}:
{
  services.postgresql = {
    package = pkgs.postgresql_14;
  };

  # Temporary: upgrade script for PostgreSQL 14 → 16 migration.
  # After upgrading, change package above to postgresql_16 and remove this block.
  environment.systemPackages = [
    (
      let
        newPostgres = pkgs.postgresql_16;
        cfg = config.services.postgresql;
      in
      pkgs.writeScriptBin "upgrade-pg-cluster" ''
        set -eux
        systemctl stop postgresql

        export NEWDATA="/var/lib/postgresql/${newPostgres.psqlSchema}"
        export NEWBIN="${newPostgres}/bin"
        export OLDDATA="${cfg.dataDir}"
        export OLDBIN="${cfg.finalPackage}/bin"

        install -d -m 0700 -o postgres -g postgres "$NEWDATA"
        cd "$NEWDATA"
        sudo -u postgres "$NEWBIN/initdb" -D "$NEWDATA" ${lib.escapeShellArgs cfg.initdbArgs}

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