Why is index-time synonym expansion considered better than query-time synonym expansion?
For a variety of reasons, it is usually better to do this at index-time: • A synonym containing multiple words (example: i pod) isn’t recognized correctly at query-time because the query parser tokenizes on whitespace. • The IDF component of Lucene’s scoring algorithm will be much higher for documents matching a synonym appearing rarely, as compared to its equivalents that are common. This reduces the scoring effectiveness. • Prefix, wildcard, and fuzzy queries aren’t analyzed, and thus won’t match synonyms. Why is index-time synonym expansion considered less flexible than query-time synonym expansion? Index-time synonym expansion is less flexible than query-time synonym expansion because with index-time synonym expansion, changes to the synonyms will require a complete re-index to take effect. Moreover, the index will get larger if you do index-time expansion. It’s plausible to imagine the issues above being rectified at some point. However, until then, index-time is usually best. Is
