postgresql: support JIT compilation for version >= 11

PostgreSQL (from 11 onwards AFAICT) supports JIT compilation of
(frequently used) queries. This should provide a speedup to some queries
where operations such as comparing rows is a bottleneck.

For JIT support we have to build PostgreSQL with with the --with-llvm
config flag and pass both clang and llvm into the build environment.
PostPostgreSQL then calls llvm-config to obtain the required compiler
flags.

nixos/tests/postgresql.nix

@@ -21,6 +21,14 @@ let
     CREATE TABLE xmltest ( doc xml );
     INSERT INTO xmltest (doc) VALUES ('<test>ok</test>'); -- check if libxml2 enabled
   '';
+
+  jit-test-sql = pkgs.writeText "postgresql-jit-test" ''
+    SET jit_above_cost = 1;
+    SET jit = 1;
+    EXPLAIN ANALYZE SELECT CONCAT('jit result = ', SUM(id)) FROM sth;
+    SELECT CONCAT('jit result = ', SUM(id)) from sth;
+  '';
+
   make-postgresql-test = postgresql-name: postgresql-package: backup-all: makeTest {
     name = postgresql-name;
     meta = with pkgs.lib.maintainers; {
@@ -76,6 +84,16 @@ let
               "stat -c '%a' /var/backup/postgresql/${backupName}.sql.gz | grep 600",
           )
 
+      ${optionalString (versionAtLeast postgresql-package.version "11") ''
+      with subtest("Postgresql JIT should work on postgresql >= 11"):
+           output = machine.succeed(
+               "cat ${jit-test-sql} | sudo -u postgres psql"
+           )
+           assert "JIT:" in output
+           assert "jit result = 5" in output
+      ''}
+
+
       with subtest("Initdb works"):
           machine.succeed("sudo -u postgres initdb -D /tmp/testpostgres2")
 

pkgs/servers/sql/postgresql/default.nix

@@ -10,6 +10,10 @@ let
       , enableSystemd ? (lib.versionAtLeast version "9.6" && !stdenv.isDarwin)
       , gssSupport ? with stdenv.hostPlatform; !isWindows && !isStatic, libkrb5
 
+      # enable JIT support
+      , supportJIT ? (lib.versionAtLeast version "11")
+      , llvm, clang
+
 
       # for postgreql.pkgs
       , this, self, newScope, buildEnv
@@ -43,7 +47,9 @@ let
       ++ lib.optionals gssSupport [ libkrb5 ]
       ++ lib.optionals (!stdenv.isDarwin) [ libossp_uuid ];
 
-    nativeBuildInputs = [ makeWrapper ] ++ lib.optionals icuEnabled [ pkg-config ];
+    nativeBuildInputs = [ makeWrapper ]
+      ++ lib.optionals icuEnabled [ pkg-config ]
+      ++ lib.optionals supportJIT [ llvm clang ];
 
     enableParallelBuilding = !stdenv.isDarwin;
 
@@ -66,7 +72,8 @@ let
       (lib.optionalString enableSystemd "--with-systemd")
       (if stdenv.isDarwin then "--with-uuid=e2fs" else "--with-ossp-uuid")
     ] ++ lib.optionals icuEnabled [ "--with-icu" ]
-      ++ lib.optionals gssSupport [ "--with-gssapi" ];
+      ++ lib.optionals gssSupport [ "--with-gssapi" ]
+      ++ lib.optionals supportJIT [ "--with-llvm" ];
 
     patches =
       [ (if atLeast "9.4" then ./patches/disable-resolve_symlinks-94.patch else ./patches/disable-resolve_symlinks.patch)