psql_database_schemas1.htm
 
    select datname from pg_database;

Others to try: 

SELECT *FROM information_schema.columns
SELECT distinct table_catalog, table_schema,table_name, column_name FROM information_schema.columns 
 

 
using pgAdmin III

Note: In pgAdmin III, you must select a Database to enable SQL Tool:

I have two Users/Owners, default is postgres.
In the query below I used postgres, default user's SQL worksheet. Both the users are allowed to access all the databases in PostgreSQL dbms