You can load-test search latency with a script and a graph. Relevance has no such gauge by default, so most teams ship a new ranking rule, eyeball a handful of queries, and hope nothing important regressed. The fix is a small, boring relevance test suite: a fixed set of queries, human-judged expected results, and a metric you compute the same way every time — so "did this ranking change help?" becomes a number you can diff, not an argument you have in Slack.
This post is a build guide. By the end you'll have a judgments file, a scorer that outputs precision@k, MRR, and nDCG, and a before/after comparison you can wire into CI. The examples use Postgres full-text search, but the harness is engine-agnostic — Elasticsearch, Meilisearch, or a vector store all slot into the same shape.
Why can't I just load-test relevance the way I load-test latency?
Latency is a property of the system. Relevance is a property of the match between a query and what a human expected to see — and that judgment lives outside the database. A commenter on an earlier post about running Postgres search in production put it well: latency can be load-tested, but quality needs query sets, expected result buckets, bad-query examples, and a way to compare changes before shipping a new ranking rule. That's the whole job, and none of it comes for free with your index.
