postgresql: support JIT

Disable it by default because of closure size, and to avoid surprise
negative performance changes in production (were observed with very
large and complex queries due to compilation time).

See also dcd9455 for a great
explanation on how JIT interacts with packaging and closure size.

pkgs/servers/sql/postgresql/default.nix

@@ -9,6 +9,8 @@ let
       # This is important to obtain a version of `libpq` that does not depend on systemd.
       , enableSystemd ? (lib.versionAtLeast version "9.6" && !stdenv.isDarwin)
       , gssSupport ? with stdenv.hostPlatform; !isWindows && !isStatic, libkrb5
+      # Disable to avoid depending on LLVM and Clang by default.
+      , jitSupport ? false, llvm, clang
 
       # for postgreql.pkgs
       , self, newScope, buildEnv
@@ -19,11 +21,15 @@ let
       # for tests
       nixosTests, thisAttr
     }:
+
   let
     atLeast = lib.versionAtLeast version;
     icuEnabled = atLeast "10";
     lz4Enabled = atLeast "14";
-    this = stdenv.mkDerivation rec {
+  in
+    assert jitSupport -> atLeast "11";
+
+    let this = stdenv.mkDerivation rec {
       pname = "postgresql";
       inherit version;
 
@@ -43,7 +49,8 @@ let
         ++ lib.optionals lz4Enabled [ lz4 ]
         ++ lib.optionals enableSystemd [ systemd ]
         ++ lib.optionals gssSupport [ libkrb5 ]
-        ++ lib.optionals (!stdenv.isDarwin) [ libossp_uuid ];
+        ++ lib.optionals (!stdenv.isDarwin) [ libossp_uuid ]
+        ++ lib.optionals jitSupport [ llvm clang ];
 
       nativeBuildInputs = [ makeWrapper ] ++ lib.optionals icuEnabled [ pkg-config ];
 
@@ -70,7 +77,8 @@ let
       ] ++ lib.optionals icuEnabled [ "--with-icu" ]
         ++ lib.optionals lz4Enabled [ "--with-lz4" ]
         ++ lib.optionals gssSupport [ "--with-gssapi" ]
-        ++ lib.optionals stdenv.hostPlatform.isRiscV [ "--disable-spinlocks" ];
+        ++ lib.optionals stdenv.hostPlatform.isRiscV [ "--disable-spinlocks" ]
+        ++ lib.optionals jitSupport [ "--with-llvm" ];
 
       patches =
         [ (if atLeast "9.4" then ./patches/disable-resolve_symlinks-94.patch else ./patches/disable-resolve_symlinks.patch)