#Different from the rest synonym update

Intuitively, if we were to represent this as a finite-state automaton, you might think that Solr is building up something like this (ignoring plurals):Īnd your poor, unlikely document must match all four terms in sequence. Huh? What’s going on here? Well, it turns out that the SynonymFilterFactory isn’t expanding your multi-word synonyms the way you might think. “breast cancer” with the quotes), your document must literally match something like “breast cancer breast breast” in order to work. However, this also means that, if you’re doing a phrase query (i.e. …which matches documents containing “breast neoplasms,” “cancer of the breast,” etc. +((breast breast breast breast breast cancer cancer) (cancer neoplasm neoplasms tumor tumors) breast breast) So in a normal SynonymFilterFactory setup with expand=”true”, a query for “breast cancer” becomes: Consider, for example, the synonyms for “breast cancer”:

MeSH is a medical ontology that works pretty well to provide some sensible synonyms for the health domain. Multi-word synonyms won’t work as phrase queriesĪt Health On the Net, our search engine uses MeSH terms for query expansion. This is kind of complicated, so it’s worth stepping through each of these problems in turn.

Multi-word synonyms won’t be matched in queries.

The IDF of rare synonyms will be boosted, causing unintuitive results.

Multi-word synonyms won’t work as phrase queries.

They explain that query-time synonym expansion has two negative side effects: However, according to the Solr docs, this is a Very Bad Thing to Do(™), and apparently you should put the SynonymFilterFactory into the index analyzer instead, despite what your instincts would tell you.

Synonyms work instantly there’s no need to re-index.

#Different from the rest synonym update

Your synonyms can be swapped out at any time, without having to update the index.In theory, this should have several advantages:

Your first, intuitive choice might be to put the SynonymFilterFactory in the query analyzer. Our problem is specific to Solr, but the choice between these two approaches can apply to any information retrieval system. The graphic below summarizes the basic differences between index-time and query-time expansion. Where it gets complicated is when you have to decide where to fit the SynonymFilterFactory: into the query analyzer or the index analyzer? Index-time vs. You can even choose whether to expand your synonyms reciprocally or to specify a particular directionality.įor instance, you can make “dog,” “hound,” and “pooch” all expand to “dog | hound | pooch,” or you can specify that “dog” maps to “hound” but not vice-versa, or you can make them all collapse to “dog.” This part of the synonym handling is very flexible and works quite well. Solr provides a cool-sounding SynonymFilterFactory, which can be a fed a simple text file containing comma-separated synonyms. And there are lots of good ways to shoot yourself in the foot. A Rover by any other name would taste just as sweet.Īs it turns out, though, Solr doesn’t make synonym expansion as easy as you might like.