如何对检索到的结果重新排序以减轻“迷失在中间”效应

随着检索文档数量的增加（例如，超过 10 个），RAG 应用程序的性能会大幅下降。简而言之：模型在长上下文中容易错过相关信息。

相比之下，针对 vector store 的查询通常会按相关性降序返回文档（例如，通过嵌入的余弦相似度来衡量）。

为了减轻“迷失在中间”的效果，您可以在检索后对文档重新排序，以便将最相关的文档置于极值（例如，上下文的第一段和最后一段），而将最不相关的文档置于中间。在某些情况下，这有助于向 LLM 显示最相关的信息。

LongContextReorder 文档转换器实现此重新排序过程。下面我们演示一个示例。

%pip install -qU langchain langchain-community langchain-openai

首先，我们嵌入一些人工文档，并在基本的内存向量存储中为它们编制索引。我们将使用 OpenAI 嵌入，但任何 LangChain 向量存储或嵌入模型都足够了。

from langchain_core.vectorstores import InMemoryVectorStore
from langchain_openai import OpenAIEmbeddings

# Get embeddings.
embeddings = OpenAIEmbeddings()

texts = [
    "Basquetball is a great sport.",
    "Fly me to the moon is one of my favourite songs.",
    "The Celtics are my favourite team.",
    "This is a document about the Boston Celtics",
    "I simply love going to the movies",
    "The Boston Celtics won the game by 20 points",
    "This is just a random text.",
    "Elden Ring is one of the best games in the last 15 years.",
    "L. Kornet is one of the best Celtics players.",
    "Larry Bird was an iconic NBA player.",
]

# Create a retriever
retriever = InMemoryVectorStore.from_texts(texts, embedding=embeddings).as_retriever(
    search_kwargs={"k": 10}
)
query = "What can you tell me about the Celtics?"

# Get relevant documents ordered by relevance score
docs = retriever.invoke(query)
for doc in docs:
    print(f"- {doc.page_content}")

API 参考：InMemoryVectorStore | OpenAI 嵌入

- The Celtics are my favourite team.
- This is a document about the Boston Celtics
- The Boston Celtics won the game by 20 points
- L. Kornet is one of the best Celtics players.
- Basquetball is a great sport.
- Larry Bird was an iconic NBA player.
- This is just a random text.
- I simply love going to the movies
- Fly me to the moon is one of my favourite songs.
- Elden Ring is one of the best games in the last 15 years.

请注意，文档按与查询相关的降序返回。这LongContextReorderDocument Transformer 将实现上述 re-order：

from langchain_community.document_transformers import LongContextReorder

# Reorder the documents:
# Less relevant document will be at the middle of the list and more
# relevant elements at beginning / end.
reordering = LongContextReorder()
reordered_docs = reordering.transform_documents(docs)

# Confirm that the 4 relevant documents are at beginning and end.
for doc in reordered_docs:
    print(f"- {doc.page_content}")

API 参考：LongContextReorder

- This is a document about the Boston Celtics
- L. Kornet is one of the best Celtics players.
- Larry Bird was an iconic NBA player.
- I simply love going to the movies
- Elden Ring is one of the best games in the last 15 years.
- Fly me to the moon is one of my favourite songs.
- This is just a random text.
- Basquetball is a great sport.
- The Boston Celtics won the game by 20 points
- The Celtics are my favourite team.

下面，我们将展示如何将重新排序的文档合并到一个简单的问答链中：

from langchain.chains.combine_documents import create_stuff_documents_chain
from langchain_core.prompts import PromptTemplate
from langchain_openai import ChatOpenAI

llm = ChatOpenAI(model="gpt-4o-mini")

prompt_template = """
Given these texts:
-----
{context}
-----
Please answer the following question:
{query}
"""

prompt = PromptTemplate(
    template=prompt_template,
    input_variables=["context", "query"],
)

# Create and invoke the chain:
chain = create_stuff_documents_chain(llm, prompt)
response = chain.invoke({"context": reordered_docs, "query": query})
print(response)

API 参考：create_stuff_documents_chain | 提示模板 | 聊天OpenAI

The Boston Celtics are a professional basketball team known for their rich history and success in the NBA. L. Kornet is recognized as one of the best players on the team, and the Celtics recently won a game by 20 points. The Celtics are favored by some fans, as indicated by the statement, "The Celtics are my favourite team." Overall, they have a strong following and are considered a significant part of basketball culture.