Q:

Which is the Postgres version

I have a doubt about my version of Postgres installed in my environment.  
This is my os: CentOS Linux release 7.3.1611 (Core)
In /usr/ I can see: pgsql-9.4/ folder. 
But when I do: select version(); I get: 
PostgreSQL 9.2.18 on x86_64-redhat-linux-gnu, compiled by gcc (GCC) 4.8.5 
20150623 (Red Hat 4.8.5-11), 64-bit

So, what version do I have? 

A:

To determine the version of the database server, use select version() (from a connection to the database):
postgres=# select version();
                                                version                                                     
----------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.2.14 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-16), 64-bit

To determine the version of the database client, use psql --version (from your shell command line):
$ psql --version
psql (PostgreSQL) 9.6.2

The server is where data is stored.  The client is the software you use to connect to the server.  They can be different versions.