su -l postgres -c pg_dumpall | xz > ~/db_`date +%Y-%m-%d:%H:%M:%S`.sql.xz
xzcat pg_dumpall.YOURTIMESTAMP.sql.xz | su -l postgres -c 'psql -f -'

psql -U postgres -d nameofyourdatabase -c 'VACUUM FULL;'

psql -U postgres -c 'SELECT * FROM pg_user;'

SELECT CURRENT_USER;
SELECT * FROM information_schema.role_table_grants WHERE grantee='myexampleuser';

ALTER USER theusername WITH ENCRYPTED PASSWORD 'thepassword';

---- list all databases
\l
---- ...with more details
\l+
---- list users
\du
---- connect to database
\c my_db_name
---- list tables of connected database
\dt
---- ...with more details
\d+
---- list columns of a table
\d+ my_table_name
---- list extensions
\dx
---- list current connections
SELECT * FROM pg_stat_activity;
---- show version information
SELECT version();