PostgreSQL cheat sheet

Reference: https://www.digitalocean.com/community/tutorials/how-to-install-and-use-postgresql-on-ubuntu-14-04

sudo -i -u postgres

psql


1. To create a user

#create user unais with password '123';

==> To make superuser

#alter user unais with superuser;
select * from sessions limit 10;
2. To create database

#create database dbname;

3. To grant all privileges on a db to a user

#grant all privileges on database dbname to dbuser;

4. To log in to a single database

psql -U unais dbname;

If peer authentication fails, edit /etc/postgresql/9.1/main/pg_hba.conf

local              all                                     postgres                     all

TO

local             all                                      postgres                     md5

Peer authentication

The peer authentication method works by obtaining the client's operating system user name from the kernel and using it as the allowed database user name (with optional user name mapping). This method is only supported on local connections.

Password authentication

The password-based authentication methods are md5 and password. These methods operate similarly except for the way that the password is sent across the connection, namely MD5-hashed and clear-text respectively.

If you are at all concerned about password "sniffing" attacks then md5 is preferred. Plain password should always be avoided if possible. However, md5 cannot be used with the db_user_namespace feature. If the connection is protected by SSL encryption then password can be used safely (though SSL certificate authentication might be a better choice if one is depending on using SSL).

And then reload PostgreSQL

5. Error while loading application over browser

psql: FATAL: Peer authentication failed for user

Change in pg_hba.conf

local  all  all  peer    TO    local  all  all  trust

6. To list all the databases

# \list

6. To list all the tables

#\dt

6. To list roles

\du

6. To give permission to a user/role on a database where another user has full permission

GRANT user1 to user2;

then check using du -

                              List of roles
 Role name |                   Attributes                   | Member of
-----------+------------------------------------------------+------------
 db1     | Superuser, Create role, Create DB, Replication | {}
 user1  | Create DB                                      | {}
 postgres  | Superuser, Create role, Create DB, Replication | {}
 user2                                                                      {user1}                                                                            

6. To list limited entry or rows

#select * from table_name limit 10;

7. To truncate a table

TRUNCATE TABLE  table_name;

8. To list size of tables of a database in descending order

SELECT relname, relpages FROM pg_class ORDER BY relpages DESC;

9. To check version 

#psql -V