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The following page has been changed by HossMan: http://wiki.apache.org/solr/AnalyzersTokenizersTokenFilters ------------------------------------------------------------------------------ 1. The Lucene !QueryParser tokenizes on white space before giving any text to the Analyzer, so if a person searches for the words `sea biscit` the analyzer will be given the words "sea" and "biscit" seperately, and will not know that they match a synonym. 1. Phrase searching (ie: `"sea biscit"`) will cause the !QueryParser to pass the entire string to the analyzer, but if the !SynonymFilter is configured to expand the synonyms, then when the !QueryParser gets the resulting list of tokens back from the Analyzer, it will construct a !MultiPhraseQuery that will not have the desired effect. This is because of the limited mechanism available for the Analyzer to indicate that two terms occupy the same position: there is no way to indicate that a "phrase" occupies the same position as a term. For our example the resulting !MultiPhraseQuery would be `"(sea | sea | seabiscuit) (biscuit | biscit)"` which would not match the simple case of "seabisuit" occuring in a document + Even when you aren't worried about multi-word synonyms, idf differences still make index time synonyms a good idea. Consider the following scenerio: + + * An index with a "text" field, which at query time uses the !SynonymFilter with the synonym `TV, Televesion` and `expand="true"` + * Many thousands of documents containing the term "text:TV" + * A few hundred documents containing the term "text:Television" + + A query for `text:TV` will expand into `(text:TV text:Television)` and the lower docFreq for `text:Television` will give the documents that match "Television" a much higher score then docs that match "TV" comparably -- which may be somewhat counter intuative to the client. Index time expansion (or reduction) will result in the same idf for all documents regardless of which term the orriginal text contained. +
