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index_field_lengths
Enables computing and storing of field lengths (both per-document and average per-index values) into the index. Optional, default is 0 (do not compute and store). Added in 2.1.1-beta.
When index_field_lengths is set to 1, indexer
will 1) create a respective length attribute for every full-text field,
sharing the same name but with _len suffix; 2) compute a field length (counted in keywords) for
every document and store in to a respective attribute; 3) compute the per-index
averages. The lengths attributes will have a special TOKENCOUNT type, but their
values are in fact regular 32-bit integers, and their values are generally
accessible.
BM25A() and BM25F() functions in the expression ranker are based
on these lengths and require index_field_lengths to be enabled.
Historically, Sphinx used a simplified, stripped-down variant of BM25 that,
unlike the complete function, did not account for document length.
(We later realized that it should have been called BM15 from the start.)
Starting with 2.1.1-beta, we added support for both a complete variant of BM25,
and its extension towards multiple fields, called BM25F. They require
per-document length and per-field lengths, respectively. Hence the additional
directive.
Example:
index_field_lengths = 1
