default search boolean operator

I have read somewhere about the search module support boolean search by default. I am expecting by default it will do an "AND" operation on all the input search terms. But my newly installation does an default "OR" operation on all input terms, thus returns numerous irrelative results. I thought default to "AND" will be much more helpful.

Does anybody have any suggestion why this is not working? Is it by design to do so or I did something wrong?

Thanks.