Readings - Relevance Feedback and Query Expansion

I found the readings this week to be informative, as I had never considered the details of how user-feedback is or could be incorporated in to IR. I specifically found the discussion of pseudo and implicit relevance feedback to be interesting. I wonder about the tradeoffs between query efficiency and retrieval success in pseudo relevance feedback given that one must, presumably, run two queries to get one result. Is  this efficiency not really an issue?

I also found the discussion of thesaurus-based query expansion to be interesting. I have seen some of this in my work with bibliographic databases, but might look into it a little more now that I understand how it works.