postgreSQL "to_tsvector" is not working with "up" and "down" keyword

I wonder why, in postgreSQL "to_tsvector" is not working with "down" and "up" keyword?

select to_tsvector('english', coalesce('down',''));
select to_tsvector('english', coalesce('up',''));

Yo开发者_Go百科u'll notice that, we are getting the blank values. Is there any work around to search the "up" and "down" keyword with "to_tsvector"?

These are stop words, words that are too common. Check the manual to see how to change the list of stop words or even create your own dictionary.