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bigram_index
Bigram indexing mode. Optional, default is none. Added in 2.1.1-beta.
Bigram indexing is a feature to accelerate phrase searches. When indexing, it stores a document list for either all or some of the adjacent words pairs into the index. Such a list can then be used at searching time to significantly accelerate phrase or sub-phrase matching.
bigram_index controls the selection of specific word pairs.
The known modes are:
all, index every single word pair. (NB: probably totally not worth it even on a moderately sized index, but added anyway for the sake of completeness.)first_freq, only index word pairs where the first word is in a list of frequent words (see the section called “bigram_freq_words”). For example, withbigram_freq_words = the, in, i, a, indexing “alone in the dark” text will result in “in the” and “the dark” pairs being stored as bigrams, because they begin with a frequent keyword (either “in” or “the” respectively), but “alone in” would not be indexed, because “in” is a second word in that pair.both_freq, only index word pairs where both words are frequent. Continuing with the same example, in this mode indexing “alone in the dark” would only store “in the” (the very worst of them all from searching perspective) as a bigram, but none of the other word pairs.
For most usecases, both_freq would be the best mode, but
your mileage may vary.
Example:
bigram_freq_words = both_freq
