Overlays in nixpkgs!

all-packages.nix

...

 inherit (import ../servers/sql/postgresql-default.nix pkgs super)
    postgresql
    postgresqlPackages
    postgresql_9_4
    postgresql_9_5
    postgresql_9_6
    postgresql_10
    postgresql_11
  ;

...

postgresql-default.nix

let
  generic = ....;
  
in self: super: {

  postgresqlPackages = self.lib.makeExtensible ...;

  postgresql = self.postgresql_9_6;

  postgresql_9_4 = self.callPackage generic {
    version = "9.4.20";
    psqlSchema = "9.4";
    sha256 = "0zzqjz5jrn624hzh04drpj6axh30a9k6bgawid6rwk45nbfxicgf";
    this = self.postgresql_9_4;
  };

  postgresql_9_5 = self.callPackage generic {
    version = "9.5.15";
    psqlSchema = "9.5";
    sha256 = "0i2lylgmsmy2g1ixlvl112fryp7jmrd0i2brk8sxb7vzzpg3znnv";
    this = self.postgresql_9_5;
  };

  postgresql_9_6 = self.callPackage generic {
    version = "9.6.11";
    psqlSchema = "9.6";
    sha256 = "0c55akrkzqd6p6a8hr0338wk246hl76r9j16p4zn3s51d7f0l99q";
    this = self.postgresql_9_6;
  };

  postgresql_10 = self.callPackage generic {
    version = "10.6";
    psqlSchema = "10.0";
    sha256 = "0jv26y3f10svrjxzsgqxg956c86b664azyk2wppzpa5x11pjga38";
    this = self.postgresql_10;
  };

  postgresql_11 = self.callPackage generic {
    version = "11.1";
    psqlSchema = "11.1";
    sha256 = "026v0sicsh7avzi45waf8shcbhivyxmi7qgn9fd1x0vl520mx0ch";
    this = self.postgresql_11;
  };
}