If you stay longer in Melk, you have to go to a wine tavern in the Wachau. Any local will be able to provide information on this as well. You only have to find out which wine grower on which side of the Danube in which place is currently open. Find your local in the calendar.
"Brettljause and Achterl" belong to the Wachau, like the Lipizzaner to the Spanish Riding School.
To be sure of finding a seat, you'd better book.