Since the current version has compromised the Classifieds searches there have been a lot of complaints (myself included). So here's a couple of hopefully constructive suggestions for fixing this since I believe that a good classified system is critical for the economy in SL.
1. Assuming that there is going to be any kind of text search capability, the db load problem must be on the db store side. If it's on the actual search side, the current solution won't scale as people just respond with more ads with keywords as the first word in the title. Given that, how about just imposing a delay on posting ads, say an hour. Then do the keyword indexing as a background task running on an hour cycle.
2. If the problem is searching for an open ended set of keywords, go to the classic newspaper classifieds ads system. Have a fixed list of classifieds catagories, hopefully hierarchical, and have people just put their ad into one of the catagories. By proper encoding, this allows very fast retrieval of the list of ads in each catagory.The search would be done as a set of filters on the catagories.
This scheme means that a single ad placement would appear in only a single catagory, but it could be placed in several of course.
It would be a nice thing if there was also an automated way for people to submit new catagories and have the encoding expanded once a week or so.