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Ranking overview
Ranking (aka weighting) of the search results can be defined as a process of computing a so-called relevance (aka weight) for every given matched document with regards to a given query that matched it. So relevance is in the end just a number attached to every document that estimates how relevant the document is to the query. Search results can then be sorted based on this number and/or some additional parameters, so that the most sought after results would come up higher on the results page.
There is no single standard one-size-fits-all way to rank any document in any scenario. Moreover, there can not ever be such a way, because relevance is subjective. As in, what seems relevant to you might not seem relevant to me. Hence, in general case it's not just hard to compute, it's theoretically impossible.
So ranking in Sphinx is configurable. It has a notion of a so-called ranker. A ranker can formally be defined as a function that takes document and query as its input and produces a relevance value as output. In layman's terms, a ranker controls exactly how (using which specific algorithm) will Sphinx assign weights to the document.
Previously, this ranking function was rigidly bound to the matching mode.
So in the legacy matching modes (that is, SPH_MATCH_ALL, SPH_MATCH_ANY,
SPH_MATCH_PHRASE, and SPH_MATCH_BOOLEAN) you can not choose the ranker.
You can only do that in the SPH_MATCH_EXTENDED mode. (Which is the only
mode in SphinxQL and the suggested mode in SphinxAPI anyway.) To choose
a non-default ranker you can either use
SetRankingMode()
with SphinxAPI, or OPTION ranker
clause in SELECT statement when using SphinxQL.
As a sidenote, legacy matching modes are internally implemented via the unified syntax anyway. When you use one of those modes, Sphinx just internally adjusts the query and sets the associated ranker, then executes the query using the very same unified code path.
