I recently ran into an issue where (after implementing fulltext search on my site) a user searching real estate listings for "no pets" also got results for "pets OK"! This was obviously a problem. After investigating, it seems the word "no" is considered a stopword by default (it's in the english.stop word list), and is therefore not indexed. I am here to propose that this is wrong based on the following reasons:
1) The word "yes" is not also included in this stopword list, a bizarre omission if the reason "no" was included was due to lack of significance (although I would recommend omitting both and arguing that both are significant)
2) The word "no" IS significant as a qualifier (such as, in my case, "no pets", or more usefully, "no<->pets" if using to_tsquery instead of plainto_tsquery), especially on boolean-like data that is brought into fulltext search scope (so for example, if some attribute "balcony" is false/not checked, you could index that as "no balcony" which then makes both the presence AND the absence of it searchable...)
That's basically it. Thoughts?
-Peter