You can add redundant columns to eliminate frequent joins.
For example, if you are performing frequent joins on the titleauthor and authors tables to retrieve the author’s last name, you can add the au_lname column to titleauthor.
Adding redundant columns eliminates joins for many queries. The problems with this solution are that it:
Requires maintenance of new columns. you must make changes to two tables, and possibly to many rows in one of the tables.
Requires more disk space, since au_lname is duplicated.