To answer that question immediately. In most cases you don’t need to Relink/Recompile PostgreSQL after an OS upgrade. However, there are exceptions, e.g. if you are using PostgreSQL with the JIT feature. Recently I did an OS Upgrade from RHEL 8.2 to 8.3 including the Kernel update from 4.18.0-193.14.3.el8_2.x86_64 to 4.18.0-240.8.1.el8_3.x86_64. What I didn’t noticed immediately, was that the llvm was upgraded as well. From Version 9.x to 10.x.
$ rpm -qa | grep llvm llvm-devel-10.0.1-3.module+el8.3.0+7719+53d428de.x86_64 llvm-10.0.1-3.module+el8.3.0+7719+53d428de.x86_64 llvm-libs-10.0.1-3.module+el8.3.0+7719+53d428de.x86_64
In case you want to learn more about LLVM you might want to check out the following page: https://llvm.org/
Remember that PostgreSQL has builtin support to perform JIT compilation using LLVM when PostgreSQL is built with –with-llvm. Check out the following page to learn more about it.
https://www.postgresql.org/docs/12/jit-reason.html
So … if you have compiled PostgreSQL with LLVM, and you upgrade your OS (which might come with new LLVM libs), you need to relink/recompile PostgreSQL. If you do not relink it, you might end up with the following ERROR’s.
2021-03-09 15:17:23.156 CET [113465] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory 2021-03-09 15:17:23.156 CET [103680] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory 2021-03-09 15:17:23.156 CET [113466] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory 2021-03-09 15:17:23.156 CET [113467] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory 2021-03-09 15:17:37.303 CET [113689] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory 2021-03-09 15:17:37.303 CET [103680] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory 2021-03-09 15:17:37.303 CET [113691] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory 2021-03-09 15:17:37.303 CET [113690] ERROR: could not load library "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so": libLLVM-9.so: cannot open shared object file: No such file or directory
By double checking it with the command ldd, you can see that the shared object libLLVM-9.so is indeed not available.
$ ldd "/app/lib/postgres/pgproduct/pg-12.4/lib/llvmjit.so"
linux-vdso.so.1 (0x00007fffaf52f000)
libLLVM-9.so => not found
libstdc++.so.6 => /lib64/libstdc++.so.6 (0x0000150578d40000)
libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x0000150578b28000)
libc.so.6 => /lib64/libc.so.6 (0x0000150578765000)
libm.so.6 => /lib64/libm.so.6 (0x00001505783e3000)
/lib64/ld-linux-x86-64.so.2 (0x00001505792f4000)
To correct this issue, you have two possibilities. The first one is to compile your software again with the new Kernel and LLVM libraries.
https://ptdb.ch/2020/10/26/how-to-compile-postgresql-13-on-centos-8-2/
Or to disable JIT by changing the following parameter in the postgresql.conf file
jit = off
Turning this option off is an online operation. It just requires a reload of your configuration.
Conclusion
In case you are not using PostgreSQL with JIT, you don’t need to relink your PostgreSQL software. However, if you are using JIT, which is available since PostgreSQL 11, you need to relink/re-compile your software, or to temporarily disable JIT.