The book states “You can explicitly add a hash index using the CREATE INDEX command, where each value must be unique (like a hashtable or a map).” and then gives you the query to create the index. I created the index and then tried inserting another row with the same title, and it allowed it. It also appears to work fine to select for these rows with the same title.
Does the hash index really enforce uniqueness? I don’t see anything about uniqueness in the docs http://www.postgresql.org/docs/9.2/static/indexes-types.html .