Related posts functionality

I am trying to build a related posts functionality like what we see here in stackoverflow on the bottom right corner. My main difficulty is that the related posts have to be determined at runtime when loading a particular post.

I am thinking to look for posts wit开发者_JAVA百科h similar or equal set of tags, also similar titles and possibly similar sets of keywords in the main post content, the problem is that the more signals you look for the slower the database will be to return data.

I have also thought about using Google Site Search (we have an account) as Google does understand the relationship between posts very well, but unfortunately the related: parameter is broken according to Google.

I am looking for ideas on how to best achieve this. Has any of you ever done something like this? What's the best way to achieve this type of thing?