Some time ago I wrote a blogpost about why index might not be used.
While this post seemed to be well received (top link from depesz.com on reddit), it doesn't answer another question – what index to create for given situation.
I'll try to cover this question now.
Friend from my previous employer told me that plans of execution of prepared statement, and the same statement run “as it" are different.
Well, I checked and this is what I found (it's not shocking, it's actually quite obvious, but You have to think about it for a while to “get it").