Query Transformation Guide

Query transformation rewrites or expands a user query before retrieval to improve recall and relevance. A single natural-language question may not align well with the embedding space; transforming it into one or more alternative phrasings helps the retriever surface better results.


User Query --> Transform --> [Query 1, Query 2, ...] --> Retrieve --> Merge

All transformers accept callback functions for LLM generation so that anchor never calls an LLM directly. You supply your own generation function and the transformer handles orchestration.

Transformer Overview

Transformer	Strategy	Output
`HyDETransformer`	Generate a hypothetical answer to the query	1 query
`MultiQueryTransformer`	Generate N alternative phrasings	N+1 queries
`DecompositionTransformer`	Break a complex query into sub-questions	N queries
`StepBackTransformer`	Generate a broader, more abstract version	2 queries
`ConversationRewriter`	Rewrite using chat history	1 query
`ContextualQueryTransformer`	Prepend conversation context, then delegate	varies

HyDETransformer

Hypothetical Document Embeddings (HyDE) generates a plausible answer to the query and uses that as the retrieval query. The intuition is that a hypothetical answer is closer in embedding space to the real answer than the question itself.

from anchor.query import HyDETransformer
from anchor.models.query import QueryBundle

def generate_hypothetical(query: str) -> str:
    # In production, call your LLM here
    return f"A hypothetical answer about: {query}"

hyde = HyDETransformer(generate_fn=generate_hypothetical)

query = QueryBundle(query_str="What is retrieval augmented generation?")
expanded = hyde.transform(query)

print(expanded[0].query_str)
# "A hypothetical answer about: What is retrieval augmented generation?"
print(expanded[0].metadata["transform"])
# "hyde"

MultiQueryTransformer

Generates multiple query variations for broader retrieval coverage. The original query is always included as the first element.

from anchor.query import MultiQueryTransformer
from anchor.models.query import QueryBundle

def generate_variations(query: str, count: int) -> list[str]:
    return [f"variation {i}: {query}" for i in range(count)]

multi = MultiQueryTransformer(generate_fn=generate_variations, num_queries=3)

query = QueryBundle(query_str="How does RAG work?")
expanded = multi.transform(query)

print(len(expanded))  # 4 (original + 3 variations)
print(expanded[0].query_str)  # "How does RAG work?" (original)

DecompositionTransformer

Breaks a complex query into simpler sub-questions. Useful for multi-hop or compound questions where answering the original requires synthesizing information from multiple sources.

from anchor.query import DecompositionTransformer
from anchor.models.query import QueryBundle

def decompose(query: str) -> list[str]:
    return [
        "What is retrieval augmented generation?",
        "What are the benefits of RAG over fine-tuning?",
    ]

decomp = DecompositionTransformer(generate_fn=decompose)

query = QueryBundle(query_str="Compare RAG and fine-tuning approaches")
sub_queries = decomp.transform(query)

for sq in sub_queries:
    print(sq.query_str)
    print(f"  parent_query: {sq.metadata['parent_query']}")

StepBackTransformer

Generates a more abstract version of the query alongside the original. The "step-back" technique retrieves high-level context that helps answer specific questions.

from anchor.query import StepBackTransformer
from anchor.models.query import QueryBundle

def step_back(query: str) -> str:
    return "What are the general principles of information retrieval?"

sb = StepBackTransformer(generate_fn=step_back)

query = QueryBundle(query_str="How does BM25 scoring work?")
expanded = sb.transform(query)

print(len(expanded))  # 2: [original, step-back]
print(expanded[0].query_str)  # "How does BM25 scoring work?"
print(expanded[1].query_str)  # "What are the general principles of information retrieval?"

Chaining Transformers with QueryTransformPipeline

QueryTransformPipeline chains multiple transformers in sequence. Each transformer is applied to every query produced by the previous stage, producing a flat list of unique queries at the end (deduplicated by query_str).

from anchor.query import (
    QueryTransformPipeline,
    MultiQueryTransformer,
    StepBackTransformer,
)
from anchor.models.query import QueryBundle

def gen_variations(query: str, count: int) -> list[str]:
    return [f"v{i}: {query}" for i in range(count)]

def gen_stepback(query: str) -> str:
    return f"General: {query}"

pipeline = QueryTransformPipeline(
    transformers=[
        MultiQueryTransformer(generate_fn=gen_variations, num_queries=2),
        StepBackTransformer(generate_fn=gen_stepback),
    ]
)

query = QueryBundle(query_str="How does vector search work?")
all_queries = pipeline.transform(query)

print(f"Produced {len(all_queries)} unique queries")
for q in all_queries:
    print(f"  - {q.query_str}")

[!NOTE] QueryTransformPipeline also supports atransform() for async execution. Transformers that implement AsyncQueryTransformer are called via atransform; others fall back to the synchronous transform method.

Conversation-Aware Transformers

ConversationRewriter

Rewrites a query using conversation history via a user-supplied callback. When the QueryBundle carries non-empty chat_history, the rewrite_fn is called. If history is empty, the original query passes through unchanged.

from anchor.query import ConversationRewriter
from anchor.models.query import QueryBundle
from anchor.models.memory import ConversationTurn

def rewrite(query: str, history: list[ConversationTurn]) -> str:
    context = "; ".join(f"{t.role}: {t.content}" for t in history)
    return f"{query} (context: {context})"

rewriter = ConversationRewriter(rewrite_fn=rewrite)

query = QueryBundle(
    query_str="What about the second approach?",
    chat_history=[
        ConversationTurn(role="user", content="Compare RAG and fine-tuning"),
        ConversationTurn(role="assistant", content="RAG retrieves documents..."),
    ],
)
result = rewriter.transform(query)
print(result[0].query_str)

ContextualQueryTransformer

Wraps another transformer, prepending a summary of conversation history to the query string before delegation.

from anchor.query import ContextualQueryTransformer, HyDETransformer
from anchor.models.query import QueryBundle
from anchor.models.memory import ConversationTurn

def gen_hyde(query: str) -> str:
    return f"Hypothetical: {query}"

inner = HyDETransformer(generate_fn=gen_hyde)
contextual = ContextualQueryTransformer(
    inner=inner,
    context_prefix="Given the conversation context: ",
)

query = QueryBundle(
    query_str="Tell me more",
    chat_history=[
        ConversationTurn(role="user", content="What is RAG?"),
    ],
)
result = contextual.transform(query)
print(result[0].query_str)
# Contains the conversation context prepended to the HyDE output

Integration with Pipeline Steps

Use query_transform_step() to plug any transformer directly into a retrieval pipeline. The step transforms the query, retrieves for each variant using the provided retriever, and merges results via Reciprocal Rank Fusion.

from anchor.query import MultiQueryTransformer
from anchor.pipeline import query_transform_step

def gen_variations(query: str, count: int) -> list[str]:
    return [f"v{i}: {query}" for i in range(count)]

transformer = MultiQueryTransformer(
    generate_fn=gen_variations,
    num_queries=3,
)

# `my_retriever` is any object implementing the Retriever protocol
step = query_transform_step(
    name="multi-query-retrieval",
    transformer=transformer,
    retriever=my_retriever,
    top_k=10,
)

The step can then be added to a ContextPipeline:

from anchor.pipeline import ContextPipeline

pipeline = ContextPipeline(steps=[step])

[!TIP] Combine QueryTransformPipeline with query_transform_step to chain multiple transformers and use the combined output for retrieval in a single pipeline step.

[!CAUTION] Each expanded query triggers a separate retrieval call. With MultiQueryTransformer(num_queries=3) you get 4 retrieval calls (original + 3 variations). Keep top_k and num_queries reasonable to control latency and cost.

Query Transformation Guide

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