Pick where to search, and see what each change bought.
Routing: pick where to search
Query routing
Deciding, per question, which source or index to search (or whether to search at all). It is an if/else decided in natural language.
Routing is about where , not what . A company may have a vector store for documentation, a SQL database for orders and a web API for live status. The example routes among three subsets of the corpus (errors, billing, how-to) by keyword votes.
Routing's weakness is built in: the router has to be right, and one route cannot answer a question that spans two . In the results, routing scores 25 percent on compound questions, because "refund" and "webhook retry" live in different subsets and the router picks one. A test checks exactly that. If you route, give the router a fallback ("search everything when unsure") and consider fanning out to several routes.
The measured effect
The example asks 16 questions at "top 2": a question counts as answered only if all its relevant passages are in the first two results (compound questions have two). The baseline is module 3's hybrid search.
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Every relevant passage found in the top 2, by kind of question (16 questions) 0% 25% 50% 75% 100% 50% 100% 50% 25% 50% 50% follow-ups (4) 75% 75% 100% 100% 75% 75% vocabulary (4) 50% 50% 50% 25% 75% 25% compound (4) 100% 100% 100% 100% 100% 100% plain (4) baseline rewrite multi-query hyde decompose route Share of questions with every relevant passage in the top 2, by kind of question (toy corpus, 16 questions)
System
Follow-up
Vocabulary
Compound
Plain
All
Model calls
Searches
baseline
50%
75%
50%
100%
69%
0
16
rewrite
100%
75%
50%
100%
81%
16
16
multi-query
50%
100%
50%
100%
75%
16
24
HyDE
25%
100%
25%
100%
62%
16
32
decompose
50%
75%
75%
100%
75%
16
20
route
50%
75%
25%
100%
62%
16
16
combined (rewrite, decompose, multi-query)
100%
100%
75%
100%
94%
52
28
What to take from it, as direction rather than as a benchmark:
Each tool fixes its own kind. Rewrite fixes follow-ups, multi-query and HyDE fix vocabulary gaps, decomposition helps compound questions. None helps the others' kinds.Two of the tools hurt overall (HyDE and routing end below baseline). A transformation is a bet about your questions; check it.Plain questions gain nothing and still pay. All systems score 100 percent on them, but the combined system spends three model calls on each. In a real product that is added latency for the easy majority.The best system is also the costliest (52 model calls for 16 questions versus none). Whether 94 percent is worth that is a product decision.
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Five things you can do to a question before it reaches the index Question + chat history Rewrite fill in "it" and "what about X?" Route pick which index to search Decompose split a compound question into parts Multi-query paraphrase the words the docs use HyDE search with a made- up answer Search then fuse the lists Each box is usually one model call. Use only the ones your questions need: every one adds latency and cost. Boxes are alternatives you can mix, not a fixed order. Routing in particular can hurt compound questions. The five transformations and where they sit before search Practical advice
Log the failures first. Look at real questions that retrieved badly and pick the transformation that matches the pattern. Do not add all five by default.Rewrite whenever there is chat history. Follow-ups are the cheapest, most common win for agents.Keep the original question in the mix. Fuse results from the original and the transformed query so a bad transformation cannot remove a good result.Make routing fail open. If the router is unsure, search everything.Route cheap work to a small model. Rewriting and routing are short, structured tasks.Evaluate per kind and per cost (module 7 builds the proper metrics). A transformation that gains two points and doubles latency is usually a bad trade.Remember who sees the text. A rewritten or hypothetical query is for the retriever only. Never present it as the user's words or as a fact.
Interview angle
"A user's follow-up questions retrieve badly. What do you do?" Rewrite the question using the conversation so the search query stands alone; then keep the original too, and fuse. Expect the follow-up: "what else can transform a query?" Name multi-query for vocabulary gaps, HyDE for question-versus-answer wording, decomposition for compound questions, and routing for choosing between sources, and say what each costs. The strong answer adds that you would measure each on your own question types, because HyDE and routing can lower quality, and that every one adds model calls and latency.
Check yourself
A user writes "how do I fix it?" after asking about error E-4021. Which transformation do you need, and what should you also keep in the search? (Rewrite, so the query stands alone. Keep the original query in the fusion as well, so a bad rewrite cannot remove a good result.)
For the curious A counting error found in the example codeThe combined system (rewrite, decompose, then multi-query for each part) was charged a flat three model calls per question. It really makes two calls plus one per part, so compound questions cost four. The count is now exact, and the total in the table is 52 instead of 48. It was also called "all four" although it combines three transformations; it is now called "combined".