Relax search queries without ruining precision

Start with strict Boolean matching, then relax terms in controlled tiers to avoid empty results while protecting top-rank relevance.

Boolean and tiered search

A reader searches for cache index , and none of our documents contains both words. Returning nothing is defensible if both words are requirements. Returning separate cache and index lessons is defensible if the reader is exploring a topic. The service needs to choose that behavior explicitly before a score decides which document looks best.

We'll keep required filters fixed, group strict and partial matches, and use term coverage or proximity when two groups are too coarse.

Requirements and preferences

wiki/inverted-index-and-posting-lists The posting-list lesson gave you posting lists and two ways to combine them. Requiring every query term uses their intersection, commonly expressed as AND. Accepting any query term uses their union, expressed as OR. These operators describe a matching condition; they do not measure how useful the resulting documents are.

An unknown token makes an AND intersection empty because its posting list is empty. An OR union can still contain documents matching the remaining tokens. That difference is often hidden behind a search box, but it changes what a successful response means. The user who typed a precise part number may be poorly served by a result that matches only its accompanying description.

A condition supplied as a structured filter deserves separate treatment. An allowed-document set represents which documents may be exposed in this exercise. It is chosen test input, not an authentication implementation. Every retrieval tier intersects its candidates with that same set, so relevance fallback cannot turn an excluded document into a permitted one.

Which conditions may relax?

Treat permissions and explicitly required filters as hard constraints. Treat an exploratory free-text query as a possible source of soft preferences when the interface says broader matches may appear. For a caller using an explicit “all words” search, keep AND semantics unless they request a broader search. A quiet fallback would make the response disagree with the request.

The alternatives are to return only strict matches, present broad matches separately, or mix all matches under one score. The last option is convenient, but a high score could put a partial match above a complete match unless the ranker also enforces the requirement. Choose separate ordered groups here because the behavior stays inspectable before we introduce numerical ranking.

Ordered fallback groups

A group that runs under a particular matching rule is a tier. Our chosen policy first collects all permitted AND matches as tier zero. It then collects permitted OR matches that have not already appeared as tier one. The displayed response always appends the second tier, even when the first is nonempty; this differs from a policy that broadens only after an empty strict result.

Within each tier, identifiers sort ascending. A document appears once, under the earliest rule it satisfies. Later scoring may change order within a tier, but it may not move a tier-one document ahead of a tier-zero document under this contract. If the product wants that movement, it has chosen a different contract and should evaluate it as such.

Choose the default from what the caller asked for:

Caller situation Default Reason --- --- --- Every submitted word is required AND only Preserve the requested condition Exploring with useful partial matches AND tier, then labeled OR tier Keep complete matches first Any term is independently sufficient OR with ranking No complete-match priority promised

One policy against several queries