when i logged in mydb, any sql command used to list all the tables in this mydb?

once i have created mydb and several relations in it,are there any sql
commands used to list all the tables in this mydb?
i noticed there are no database( pg_database.oid) field in pg_class table,so
i can not use
select relname from pg_class,pg_database where pg_database.datname like
'mydb' and pg_class.database = pg_database.oid;
anybody knows how to do it?
another question:how postgresql internal knows which relations belongs to
which database?

thanks