1. A computer-implemented method comprising: receiving, by a search system, from a user a search query comprising keywords; using at least one association graph comprising keywords, identifying, by the search system, one or more suggested replacement keywords for one or more of the keywords of the search query; presenting the suggested replacement keywords to the user; responsively to a selection of one of the suggested replacement keywords by the user, substituting, by the search system, the selected suggested replacement keyword for the corresponding one of the keywords of the search query, to generate a refined search query; and presenting search results to the user responsively to the refined search query, wherein identifying the one or more suggested replacement keywords comprises: designating, by the search system, one or more of the keywords of the search query as anchor keywords, and the remaining keywords of the search query as non-anchor keywords; and identifying, by the search system, the one or more suggested replacement keywords for one or more of the non-anchor keywords and not for any of the anchor keywords.