How to upgrade postgresql in NixOS? - postgresql

Question

I am attempting to update my postgresql server from 9.4 to (at least) 9.6 on my NixOS machine.
I have edited services.postgres.package in my configuration.nix to reflect this change, changing it from:
services.postgresql.package = pkgs.postgresql94
to
services.postgresql.package = pkgs.postgresql96
However, this results in an error upon running nixos-rebuild switch, namely:
$ sudo nixos-rebuild switch
building Nix...
building the system configuration...
stopping the following units: postgresql.service
NOT restarting the following changed units: display-manager.service
activating the configuration...
setting up /etc...
setting up tmpfiles
reloading the following units: dbus.service
restarting the following units: polkit.service
starting the following units: postgresql.service
Job for postgresql.service failed because the control process exited with error code.
See "systemctl status postgresql.service" and "journalctl -xe" for details.
warning: the following units failed: postgresql.service
● postgresql.service - PostgreSQL Server
Loaded: loaded (/nix/store/bh7vzvacc9y56w0kzs1mwgb1jy9bwvf6-unit-postgresql.service/postgresql.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Sat 2018-08-04 17:39:33 UTC; 26ms ago
Process: 25399 ExecStartPost=/nix/store/hj8lfb9bbspn76nwm0qmx0xr4466gh0a-unit-script/bin/postgresql-post-start (code=exited, status=1/FAILURE)
Process: 25398 ExecStart=/nix/store/qhdnk3qsw00igzadqfxf7kpp3a48z368-unit-script/bin/postgresql-start (code=exited, status=1/FAILURE)
Process: 25395 ExecStartPre=/nix/store/qg6s6mph3jmrsgr67vh4bsydxrrbmvrr-unit-script/bin/postgresql-pre-start (code=exited, status=0/SUCCESS)
Main PID: 25398 (code=exited, status=1/FAILURE)
Aug 04 17:39:33 nixos systemd[1]: Starting PostgreSQL Server...
Aug 04 17:39:33 nixos systemd[1]: postgresql.service: Main process exited, code=exited, status=1/FAILURE
Aug 04 17:39:33 nixos systemd[1]: postgresql.service: Control process exited, code=exited status=1
Aug 04 17:39:33 nixos systemd[1]: Failed to start PostgreSQL Server.
Aug 04 17:39:33 nixos systemd[1]: postgresql.service: Unit entered failed state.
Aug 04 17:39:33 nixos systemd[1]: postgresql.service: Failed with result 'exit-code'.
warning: error(s) occurred while switching to the new configuration
I notice that the NixOS manual contains a PostgreSQL section, however the "Upgrading" subsection is not yet filled out. Any ideas on how I might resolve this error and upgrade my PostgreSQL?

Answer 1

I solved this issue by creating a dump of all server databases, whacking the old data_directory and uninstalling the old version, installing the new version, then restoring from the dump.
These steps are described in detail below.
Create a dump of all server databases.
$ pg_dumpall -U root > sql-dump
Identify the location of the current version's data_directory.
root=# SHOW data_directory;
data_directory
--------------------
/var/db/postgresql
(1 row)
Change the version of services.postgresql.package in /etc/nixos/configuration.nix.
services.postgresql.package = pkgs.postgresql100
This is apparently the expression for version 10.4, according to $ nix-env -qaP '*' --description.
Next whack the data_directory for the current version.
$ sudo rm -rf /var/db/postgresql/
And switch to the new version marked in configuration.nix
$ sudo nixos-rebuild switch
I had to create a root db.
$ sudo createdb root
(I also had to change some instances of postgres to root in my sql-dump file.)
Restore the data into the new version.
$ psql -U root -f sql-dump
Anyone know how one can contribute to the nixos manual?
I am happy to use what I've learned here to write up the updating postgres section.