Relevance, facets, ranking signals, merchandising rules and zero-result handling.
3 questions in E-commerce & Retail.
With the zero-result queries, sorted by volume. Each one is a customer stating exactly what they wanted and being refused, the causes are mundane and fixable, and it is the only part of relevance work where you know for certain the current behaviour is wrong.
Segment the query log first, then blend textual match with behavioural and business signals into a ranking you can explain, treat synonyms and merchandising overrides as owned content with expiry, and handle zero results as a designed page - while guarding against the click feedback loop.
Separate the three cases first, because they need different treatment. Availability is a per-request fact rather than an index field, so filter at query time from a fast availability source and reserve suppression for what is genuinely delisted.