What's the difference between psql -V and apt-cache policy postgresql?

Question

I have installed postgres 9.6, If you type in terminal psql -V or psql --version, you know which postgresql version you have installed, however, if you type sudo apt-cache policy postgresql, it appears that you do not have installed postgresql and that the candidate is postgresql 10.

Do you know why this last command does not recognize postgres 9.6 install and which is the difference?

Thanks is advance

Answer 1

As has been mentioned in the comments, apt-cache policy <package name> asks apt-get to shows the package information, while psql -V makes the postgres itself report the version. So postgresql-9.6 is the package that contains the actual app and related files, among which psql. Either one is valid, but in case of apt-cache you need to provide the name of the actual package you've installed, while app or command might be different as in psql case.

As mentioned in the comments as well, you've installed the package as sudo apt-get install postgresql-9.6, and same name should be used with apt-cache policy:

$ apt-cache policy postgresql-9.6
postgresql-9.6:
  Installed: 9.6.3-4+b1
  Candidate: 9.6.3-4+b1
  Version table:
 *** 9.6.3-4+b1 500
<removed sources>