Установка базы данных PG на ubuntu

Я пытаюсь установить PG на мою систему ubuntu, но я не могу пройти эту ошибку для sudo apt-get install postgresql-9.1:

Package postgresql-9.1 is not available, but is referred to by another package. 
This may mean that the package is missing, has been obsoleted, or is only available
from another source

или эту ошибку для sudo apt-get install postgresql-9.4:

или эта ошибка для sudo apt-get install postgresql postgresql-contrib:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 postgresql : Depends: postgresql-9.3 but it is not going to be installed
 postgresql-contrib : Depends: postgresql-contrib-9.3 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Кто-нибудь знает, почему это или что я должен сделать, чтобы исправить это? (Раньше я sudo apt-get update).