癥狀：
[octopus@pc3 ~]$ sudo service postgresql start
Redirecting to /bin/systemctl start  postgresql.service
Job for postgresql.service failed. See "systemctl status postgresql.service" and "journalctl -xe" for details.

癥狀2：
[octopus@pc3 ~]$ psql
psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

其實問題的根源是我今天在執行sudo dnf update時無意中發現的：
--------------------------------------------------------------------------------
Total                                           116 kB/s |  75 MB     10:58    
Delta RPMs reduced 258.5 MB of updates to 74.7 MB (71.1% saved)
Running transaction check
Transaction check succeeded.
Running transaction test
Error: Transaction check error:
  file /usr/share/doc/postgresql/README.rpm-dist conflicts between attempted installs of postgresql-9.4.4-1.fc22.i686 and postgresql-9.4.4-1.fc22.x86_64

Error Summary
-------------
這是因為出現了conflicts，系統中同時安裝了postgresql-9.4.4-1.fc22.i686 and postgresql-9.4.4-1.fc22.x86_64兩個版本而出錯。因此只需卸載其中一個版本即可。

[octopus@pc3 ~]$ sudo dnf remove postgresql-9.4.3-1.fc22.x86_64
Dependencies resolved.
================================================================================
 Package                  Arch         Version              Repository     Size
================================================================================
Removing:
 postgresql               x86_64       9.4.3-1.fc22         @System       3.8 M
 postgresql-devel         x86_64       9.4.3-1.fc22         @System       4.3 M
 postgresql-docs          x86_64       9.4.3-1.fc22         @System        23 M
 postgresql-server        x86_64       9.4.3-1.fc22         @System        17 M
 postgresql-upgrade       x86_64       9.4.3-1.fc22         @System       9.1 M

Transaction Summary
================================================================================
Remove  5 Packages

Installed size: 58 M
Is this ok [y/N]: n
Operation aborted.

[octopus@pc3 ~]$ sudo dnf remove postgresql-9.4.3-1.fc22.i686
Dependencies resolved.
================================================================================
 Package             Arch          Version               Repository        Size
================================================================================
Removing:
 postgresql          i686          9.4.3-1.fc22          @System          3.7 M

Transaction Summary
================================================================================
Remove  1 Package

Installed size: 3.7 M
Is this ok [y/N]: y
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Erasing     : postgresql-9.4.3-1.fc22.i686                                1/1
  Verifying   : postgresql-9.4.3-1.fc22.i686                                1/1

Removed:
  postgresql.i686 9.4.3-1.fc22                                                  

Complete!

因為卸載postgresql-9.4.3-1.fc22.i686時只卸載一樣東西，而卸載postgresql-9.4.3-1.fc22.x86_64需要卸載所有的有關psql的東西。所以只卸載i686那個不完整版本。執行：
sudo dnf remove postgresql-9.4.3-1.fc22.i686

然後升級資料庫：
[octopus@pc3 ~]$ sudo postgresql-setup --upgrade
 * Upgrading database.
 * Upgraded OK.
WARNING: The configuration files were replaced by default configuration.
WARNING: The previous configuration and data are stored in folder
WARNING: /var/lib/pgsql/data-old.
 * See /var/lib/pgsql/upgrade_postgresql.log for details.

[octopus@pc3 ~]$ sudo service postgresql start
Redirecting to /bin/systemctl start  postgresql.service
啟動成功！問題已解決

配置文件已經被覆蓋。所以需要重新改配置文件。