How to develop a PostgreSQL extension on NixOS?

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Update: I’ve gotten it mostly working with this shell.nix:

{ pkgs ? import <nixpkgs> {} }:

let 
  myExtension = pkgs.stdenv.mkDerivation rec {
    pname = "myExtension";
    version = "0.1.0";

    src = ./.;

    buildInputs = [ pkgs.postgresql ];

    makeFlags = [
      "USE_PGXS=1"
    ];

    installPhase = ''
      mkdir -p $out/lib
      mkdir -p $out/share/postgresql/extension
      install -D ${pname}.so -t $out/lib
      install -D ${pname}.control -t $out/share/postgresql/extension
      install -D ${pname}--${version}.sql -t $out/share/postgresql/extension
    '';
  };

  postgresqlSrc = pkgs.fetchurl {
    url = "https://ftp.postgresql.org/pub/source/v${pkgs.postgresql.version}/postgresql-${pkgs.postgresql.version}.tar.bz2";
    sha256 = "sha256-j1OqldeOuOglNupGtoGHeTtCu6O09lqjQvVAsjybEKY=";
  };

  modifiedPostgresql = pkgs.postgresql.overrideAttrs (old: {
    src = postgresqlSrc;

    configureFlags = old.configureFlags ++ [
      "--with-includes=${myExtension}/include"
      "--with-libraries=${myExtension}/lib"
    ];

    postInstall = ''
      ${old.postInstall or ""}
      # Copy over the extension files
      mkdir -p $out/share/postgresql/extension
      mkdir -p $lib/lib
      cp -r ${myExtension}/share/postgresql/extension/* $out/share/postgresql/extension
      cp -r ${myExtension}/lib/* $lib/lib
    '';
    # postInstall = ''
    #   ${old.postInstall or ""}
    #   # Copy over the extension files
    #   mkdir -p $out/share/postgresql/extension
    #   cp -r ${myExtension}/share/postgresql/extension/* $out/share/postgresql/extension
    # '';
  });
in
pkgs.mkShell {
  buildInputs = [ modifiedPostgresql ];
  shellHook = ''
    export PATH=${modifiedPostgresql}/bin:$PATH
  '';
}

Makefile:

EXTENSION = myExtension
DATA = myExtension--0.1.0.sql
# Check if nix exists, and set PG_CONFIG accordingly
IS_NIX := $(shell which nix > /dev/null 2>&1 && echo yes || echo no)
ifeq ($(IS_NIX),yes)
   PG_CONFIG = ./pg_config_nix
else
   PG_CONFIG = pg_config
endif
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)

pg_config_nix:

#!/usr/bin/env bash

# pg_config on NixOS returns some wrong outputs
# this wrapper script aims to correct that

set -euo pipefail

INCLUDEDIR=$(nix-store -r $(which postgres) 2>/dev/null)/include

if [[ $# -eq 0 ]]; then
    pg_config | grep -v INCLUDEDIR
    echo "INCLUDEDIR = $INCLUDEDIR"
    echo "PKGINCLUDEDIR = $INCLUDEDIR"
    echo "INCLUDEDIR-SERVER = $INCLUDEDIR/server"
    exit 0
fi

# Handle specific arguments
for arg in "$@"
do
    case $arg in
        --includedir) echo $INCLUDEDIR; break;;
        --pkgincludedir) echo $INCLUDEDIR; break;;
        --includedir-server) echo "$INCLUDEDIR/server"; break;;
        *) pg_config $arg; break;;
    esac
done

However, every time I want to test the extension again the entirety of postgresql needs to be rebuilt. Still working on a solution for that. Would appreciate any help!