PostgreSQL Connect Database

• A database is a named collection of SQL objects ("database objects LIKE tables, functions,etc") that belongs to only one database. 
• But there are a few system catalogs eg - pg_database, which belong to a whole cluster and are accessible from each database within the cluster.

Accurately, a database is a collection of schemas
Schemas contain the tables, functions, etc...
hierarchy is Server, database, schema, table

• a client must specify its connection request and the name of the database it wants to connect to. It is not possible to access more than one database per connection.
• But an application is not restricted in the number of connections it opens to the same or other databases. 
• Databases arc physically separated and access control is managed at the connection level. If one PostgreSQL server instance is to house projects or users that should be separate and for the most part unaware of each other,
• it is therefore recommendable to put them into separate databases.
• If the projects or users are interraelated and hould be able to use each others resources they should be put in the same database, but possibly into separate schemas.
• Schemas is logical structure for who can access what is managed by the privilege system.

Check Whether Our Postgres Server is run or not:

[postgres@r1 bin]$ ps -ef|grep post
root      5073  4719  0 14:24 pts/1    00:00:00 su - postgres
postgres  5074  5073  0 14:24 pts/1    00:00:00 -bash
root      5149  4996  0 14:27 pts/2    00:00:00 su - postgres
postgres  5150  5149  0 14:27 pts/2    00:00:00 -bash
postgres  5188     1  0 14:28 pts/2    00:00:00 /opt/PostgreSQL/9.3/bin/postgres -D /opt/PostgreSQL/9.3/data
postgres  5189  5188  0 14:28 ?        00:00:00 postgres: logger process                                    
postgres  5191  5188  0 14:28 ?        00:00:00 postgres: checkpointer process                              
postgres  5192  5188  0 14:28 ?        00:00:00 postgres: writer process                                    
postgres  5193  5188  0 14:28 ?        00:00:00 postgres: wal writer process                                
postgres  5194  5188  0 14:28 ?        00:00:00 postgres: autovacuum launcher process                       
postgres  5195  5188  0 14:28 ?        00:00:00 postgres: stats collector process                           
postgres  5215  5074  0 14:29 pts/1    00:00:00 ps -ef
postgres  5216  5074  0 14:29 pts/1    00:00:00 grep post

Connect The Postgres Server:

[root@r1 data]# su - postgres  

[postgres@r1 bin]$ ./psql -p 5432 -U postgres -d postgres
Password for user postgres: 
psql.bin (9.3.14)
Type "help" for help.

No entry for terminal type "xterm";
using dumb terminal settings.
postgres=# 

Conform to connect the right port:

postgres=# select inet_server_port();
 inet_server_port
------------------
5432
(1 row)

Conform to connected database:

postgres=# select current_database();
 current_database 
------------------
 postgres
(1 row)

Conform to connect the right user:

postgres=# select current_user;
 current_user 
--------------
 postgres
(1 row)

Conform to connected ip address:

postgres=# select inet_server_addr();
 inet_server_addr
------------------
192.168.1.112
(1 row)

Check what Postgresql version you are using:

postgres=# select version();
                       version                        
 ---------------------------------------------------------------------------------------------------------
 PostgreSQL 9.3.14 on i686-pc-linux-gnu, compiled by gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52), 32-bit
(1 row)