As you might already know PostgreSQL 11 will bring support for just-in-time compilation. When you want to compile PostgreSQL 11 with jit support on RedHat/CentOS 7 this requires a little hack (more on the reason below). In this post we’ll look at how you can do it at least for testing. For production it is of course not recommended as hacking the make file is nothing you want to do, at least I would not do it. Lets go.

As mentioned I am on CentOS 7:

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postgres@pgbox:$ cat /etc/centos-release
CentOS Linux release 7.5.1804 (Core)

What you need to get support for jit is llvm. When you check the CentOS repository llvm is there:

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postgres@pgbox:$ yum search llvm
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirror.spreitzer.ch
 * extras: mirror.spreitzer.ch
 * updates: mirror.spreitzer.ch
===================================================== N/S matched: llvm =====================================================
llvm-devel.x86_64 : Libraries and header files for LLVM
llvm-doc.noarch : Documentation for LLVM
llvm-libs.x86_64 : LLVM shared libraries
llvm-ocaml.x86_64 : OCaml binding for LLVM
llvm-ocaml-devel.x86_64 : Development files for llvm-ocaml
llvm-ocaml-doc.noarch : Documentation for LLVM's OCaml binding
llvm-private.i686 : llvm engine for Mesa
llvm-private.x86_64 : llvm engine for Mesa
llvm-private-devel.i686 : Libraries and header files for LLVM
llvm-private-devel.x86_64 : Libraries and header files for LLVM
llvm-static.x86_64 : LLVM static libraries
mesa-private-llvm.i686 : llvm engine for Mesa
mesa-private-llvm.x86_64 : llvm engine for Mesa
mesa-private-llvm-devel.i686 : Libraries and header files for LLVM
mesa-private-llvm-devel.x86_64 : Libraries and header files for LLVM
clang.x86_64 : A C language family front-end for LLVM
llvm.x86_64 : The Low Level Virtual Machine
 
  Name and summary matches only, use "search all" for everything.

The issue is that the PostgreSQL documentation clearly states that llvm needs to be at least of version 3.9 and in the CentOS repository you’ll find this:

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postgres@pgbox:$ yum info llvm
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirror1.hs-esslingen.de
 * extras: mirror.fra10.de.leaseweb.net
 * updates: mirror.netcologne.de
Available Packages
Name        : llvm
Arch        : x86_64
Version     : 3.4.2
Release     : 8.el7
Size        : 1.3 M
Repo        : extras/7/x86_64
Summary     : The Low Level Virtual Machine
URL         : http://llvm.org/
License     : NCSA
Description : LLVM is a compiler infrastructure designed for compile-time,
            : link-time, runtime, and idle-time optimization of programs from
            : arbitrary programming languages.  The compiler infrastructure includes
            : mirror sets of programming tools as well as libraries with equivalent
            : functionality.

What to do? What you need to do is to add the epel repository where you can find llvm 3.9 and 5.0:

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postgres@pgbox:$ wget http://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
postgres@pgbox:$ sudo yum localinstall epel-release-latest-7.noarch.rpm
postgres@pgbox:$ sudo yum install llvm5.0 llvm5.0-devel clang

Having that we should be ready for configuration:

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postgres@pgbox:$ PGHOME=/u01/app/postgres/product/11/db_0
postgres@pgbox:$ SEGSIZE=1
postgres@pgbox:$ BLOCKSIZE=8
postgres@pgbox:$ WALSEGSIZE=16
postgres@pgbox:$ ./configure --prefix=${PGHOME}
            --exec-prefix=${PGHOME}
            --bindir=${PGHOME}/bin
            --libdir=${PGHOME}/lib
            --sysconfdir=${PGHOME}/etc
            --includedir=${PGHOME}/include
            --datarootdir=${PGHOME}/share
            --datadir=${PGHOME}/share
            --with-pgport=5432
            --with-perl
            --with-python
            --with-openssl
            --with-pam
            --with-ldap
            --with-libxml
            --with-libxslt
            --with-segsize=${SEGSIZE}
            --with-blocksize=${BLOCKSIZE}
            --with-wal-segsize=${WALSEGSIZE} 
            --with-llvm LLVM_CONFIG='/usr/lib64/llvm3.9/bin/llvm-config'
            --with-systemd

That succeeds so lets compile:

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postgres@pgbox:$ make -j 4 all

… and you will run into this issue:

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/usr/bin/clang -Wno-ignored-attributes -fno-strict-aliasing -fwrapv -O2  -I../../src/include  -D_GNU_SOURCE  -flto=thin -emit-llvm -c -o localtime.bc localtime.c
clang: error: unknown argument: '-flto=thin'
make[2]: *** [localtime.bc] Error 1
make[2]: Leaving directory `/home/postgres/postgresql/src/timezone'
make[1]: *** [all-timezone-recurse] Error 2
make[1]: Leaving directory `/home/postgres/postgresql/src'
make: *** [all-src-recurse] Error 2

There is a mail thread on the hackers mailing list which describes the issue. Apparently the clang compiler is too old to understand this argument. What you could do is to adjust the corresponding line in the Makefile:

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postgres@pgbox:$ vi src/Makefile.global.in
COMPILE.c.bc = $(CLANG) -Wno-ignored-attributes $(BITCODE_CFLAGS) $(CPPFLAGS) -flto=thin -emit-llvm -c
COMPILE.c.bc = $(CLANG) -Wno-ignored-attributes $(BITCODE_CFLAGS) $(CPPFLAGS) -emit-llvm -c

Doing all the stuff again afterwards:

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postgres@pgbox:$ make clean
postgres@pgbox:$ ./configure --prefix=${PGHOME}
            --exec-prefix=${PGHOME}
            --bindir=${PGHOME}/bin
            --libdir=${PGHOME}/lib
            --sysconfdir=${PGHOME}/etc
            --includedir=${PGHOME}/include
            --datarootdir=${PGHOME}/share
            --datadir=${PGHOME}/share
            --with-pgport=5432
            --with-perl
            --with-python
            --with-openssl
            --with-pam
            --with-ldap
            --with-libxml
            --with-libxslt
            --with-segsize=${SEGSIZE}
            --with-blocksize=${BLOCKSIZE}
            --with-wal-segsize=${WALSEGSIZE} 
            --with-llvm LLVM_CONFIG='/usr/lib64/llvm3.9/bin/llvm-config'
            --with-systemd
postgres@pgbox:$ make -j 4 all

… led to the following issue (at least for me):

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make[2]: g++: Command not found
make[2]: *** [llvmjit_error.o] Error 127
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory `/home/postgres/postgresql/src/backend/jit/llvm'
make[1]: *** [all-backend/jit/llvm-recurse] Error 2
make[1]: Leaving directory `/home/postgres/postgresql/src'
make: *** [all-src-recurse] Error 2

This should be easy to fix:

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postgres@pgbox:$ sudo yum install -y gcc-c++
postgres@pgbox:$ which g++
/bin/g++

Again:

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postgres@pgbox:$ make -j 4 install
postgres@pgbox:$ cd contrib
postgres@pgbox:$ make -j 4 install

… and this time it succeeds (note that I did not run the regression tests, so maybe something will still go wrong there).

JIT is enabled by default and controlled by these parameters:

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postgres=# select name,setting from pg_settings where name like 'jit%';
          name           | setting
-------------------------+---------
 jit                     | on
 jit_above_cost          | 100000
 jit_debugging_support   | off
 jit_dump_bitcode        | off
 jit_expressions         | on
 jit_inline_above_cost   | 500000
 jit_optimize_above_cost | 500000
 jit_profiling_support   | off
 jit_provider            | llvmjit
 jit_tuple_deforming     | on
(10 rows)

To test that it really kicks in you can do something like this:

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postgres=#create table ttt (a int, b text, c date );
postgres=#insert into ttt (a,b,c)
           select aa.*, md5(aa::text), now()
             from generate_series(1,1000000) aa;
postgres=#set jit_above_cost=5;
postgres=#set jit_optimize_above_cost=5;
postgres=#set jit_inline_above_cost=5;
postgres=#explain select sum(a) from ttt;

… which should lead to a plan like this:

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                                      QUERY PLAN                                      
---------------------------------------------------------------------------------------
 Finalize Aggregate  (cost=15554.55..15554.56 rows=1 width=8)
   ->  Gather  (cost=15554.33..15554.54 rows=2 width=8)
         Workers Planned: 2
         ->  Partial Aggregate  (cost=14554.33..14554.34 rows=1 width=8)
               ->  Parallel Seq Scan on ttt  (cost=0.00..13512.67 rows=416667 width=4)
 JIT:
   Functions: 8
   Inlining: true
   Optimization: true
(9 rows)

Hope that helps.

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Daniel Westermann