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Expression based ranker (SPH_RANK_EXPR)
Expression ranker, added in version 2.0.2-beta, lets you change the ranking formula on the fly, on a per-query basis. For a quick kickoff, this is how you emulate PROXIMITY_BM25 ranker using the expression based one:
SELECT *, WEIGHT() FROM myindex WHERE MATCH('hello world')
OPTION ranker=expr('sum(lcs*user_weight)*1000+bm25')
The output of this query must not change if you omit the OPTION
clause, because the default ranker (PROXIMITY_BM25) behaves exactly like
specified in the ranker formula above. But the expression ranker is somewhat
more flexible than just that and provides access to many more factors.
The ranking formula is an arbitrary arithmetic expression that can use constants, document attributes, built-in functions and operators (described in the section called “Expressions, functions, and operators”), and also a few ranking-specific things that are only accessible in a ranking formula. Namely, those are field aggregation functions, field-level, and document-level ranking factors.
