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Index sorting can be useful in order to organize Lucene doc ids (not to be
_id) in a way that makes conjunctions (a AND b AND …) more
efficient. In order to be efficient, conjunctions rely on the fact that if any
clause does not match, then the entire conjunction does not match. By using
index sorting, we can put documents that do not match together, which will
help skip efficiently over large ranges of doc IDs that do not match the
This trick only works with low-cardinality fields. A rule of thumb is that
you should sort first on fields that both have a low cardinality and are
frequently used for filtering. The sort order (
desc) does not
matter as we only care about putting values that would match the same clauses
close to each other.
For instance if you were indexing cars for sale, it might be interesting to sort by fuel type, body type, make, year of registration and finally mileage.