Create MultiQueryRetriever (#6833)

Distance-based vector database retrieval embeds (represents) queries in
high-dimensional space and finds similar embedded documents based on
"distance". But, retrieval may produce difference results with subtle
changes in query wording or if the embeddings do not capture the
semantics of the data well. Prompt engineering / tuning is sometimes
done to manually address these problems, but can be tedious.

The `MultiQueryRetriever` automates the process of prompt tuning by
using an LLM to generate multiple queries from different perspectives
for a given user input query. For each query, it retrieves a set of
relevant documents and takes the unique union across all queries to get
a larger set of potentially relevant documents. By generating multiple
perspectives on the same question, the `MultiQueryRetriever` might be
able to overcome some of the limitations of the distance-based retrieval
and get a richer set of results.

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

docs/extras/modules/data_connection/retrievers/how_to/MultiQueryRetriever.ipynb

@@ -0,0 +1,214 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "8cc82b48",
+   "metadata": {},
+   "source": [
+    "# MultiQueryRetriever\n",
+    "\n",
+    "Distance-based vector database retrieval embeds (represents) queries in high-dimensional space and finds similar embedded documents based on \"distance\". But, retrieval may produce difference results with subtle changes in query wording or if the embeddings do not capture the semantics of the data well. Prompt engineering / tuning is sometimes done to manually address these problems, but can be tedious.\n",
+    "\n",
+    "The `MultiQueryRetriever` automates the process of prompt tuning by using an LLM to generate multiple queries from different perspectives for a given user input query. For each query, it retrieves a set of relevant documents and takes the unique union across all queries to get a larger set of potentially relevant documents. By generating multiple perspectives on the same question, the `MultiQueryRetriever` might be able to overcome some of the limitations of the distance-based retrieval and get a richer set of results."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "id": "c2f3f5f2",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# Build a sample vectorDB\n",
+    "from langchain.vectorstores import Chroma\n",
+    "from langchain.document_loaders import PyPDFLoader\n",
+    "from langchain.embeddings.openai import OpenAIEmbeddings\n",
+    "from langchain.text_splitter import RecursiveCharacterTextSplitter\n",
+    "\n",
+    "# Load PDF\n",
+    "path=\"path-to-files\"\n",
+    "loaders = [\n",
+    "    PyPDFLoader(path+\"docs/cs229_lectures/MachineLearning-Lecture01.pdf\"),\n",
+    "    PyPDFLoader(path+\"docs/cs229_lectures/MachineLearning-Lecture02.pdf\"),\n",
+    "    PyPDFLoader(path+\"docs/cs229_lectures/MachineLearning-Lecture03.pdf\")\n",
+    "]\n",
+    "docs = []\n",
+    "for loader in loaders:\n",
+    "    docs.extend(loader.load())\n",
+    "    \n",
+    "# Split\n",
+    "text_splitter = RecursiveCharacterTextSplitter(chunk_size = 1500,chunk_overlap = 150)\n",
+    "splits = text_splitter.split_documents(docs)\n",
+    "\n",
+    "# VectorDB\n",
+    "embedding = OpenAIEmbeddings()\n",
+    "vectordb = Chroma.from_documents(documents=splits,embedding=embedding)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "cca8f56c",
+   "metadata": {},
+   "source": [
+    "`Simple usage`\n",
+    "\n",
+    "Specify the LLM to use for query generation, and the retriver will do the rest."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "id": "edbca101",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.chat_models import ChatOpenAI\n",
+    "from langchain.retrievers.multi_query import MultiQueryRetriever\n",
+    "question=\"What does the course say about regression?\"\n",
+    "num_queries=3\n",
+    "llm = ChatOpenAI(temperature=0)\n",
+    "retriever_from_llm = MultiQueryRetriever.from_llm(retriever=vectordb.as_retriever(),llm=llm)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "id": "e5203612",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "INFO:root:Generated queries: [\"1. What is the course's perspective on regression?\", '2. How does the course discuss regression?', '3. What information does the course provide about regression?']\n"
+     ]
+    },
+    {
+     "data": {
+      "text/plain": [
+       "6"
+      ]
+     },
+     "execution_count": 4,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "unique_docs = retriever_from_llm.get_relevant_documents(question=\"What does the course say about regression?\")\n",
+    "len(unique_docs)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "c54a282f",
+   "metadata": {},
+   "source": [
+    "`Supplying your own prompt`\n",
+    "\n",
+    "You can also supply a prompt along with an output parser to split the results into a list of queries."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 5,
+   "id": "d9afb0ca",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from typing import List\n",
+    "from langchain import LLMChain\n",
+    "from pydantic import BaseModel, Field\n",
+    "from langchain.prompts import PromptTemplate\n",
+    "from langchain.output_parsers import PydanticOutputParser\n",
+    "\n",
+    "# Output parser will split the LLM result into a list of queries\n",
+    "class LineList(BaseModel):\n",
+    "    # \"lines\" is the key (attribute name) of the parsed output\n",
+    "    lines: List[str] = Field(description=\"Lines of text\")\n",
+    "\n",
+    "class LineListOutputParser(PydanticOutputParser):\n",
+    "    def __init__(self) -> None:\n",
+    "        super().__init__(pydantic_object=LineList)\n",
+    "    def parse(self, text: str) -> LineList:\n",
+    "        lines = text.strip().split(\"\\n\")\n",
+    "        return LineList(lines=lines)\n",
+    "\n",
+    "output_parser = LineListOutputParser()\n",
+    "    \n",
+    "QUERY_PROMPT = PromptTemplate(\n",
+    "    input_variables=[\"question\"],\n",
+    "    template=\"\"\"You are an AI language model assistant. Your task is to generate five \n",
+    "    different versions of the given user question to retrieve relevant documents from a vector \n",
+    "    database. By generating multiple perspectives on the user question, your goal is to help\n",
+    "    the user overcome some of the limitations of the distance-based similarity search. \n",
+    "    Provide these alternative questions seperated by newlines.\n",
+    "    Original question: {question}\"\"\",\n",
+    ")\n",
+    "llm = ChatOpenAI(temperature=0)\n",
+    "\n",
+    "# Chain\n",
+    "llm_chain = LLMChain(llm=llm,prompt=QUERY_PROMPT,output_parser=output_parser)\n",
+    " \n",
+    "# Other inputs\n",
+    "question=\"What does the course say about regression?\""
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "id": "6660d7ee",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "INFO:root:Generated queries: [\"1. What is the course's perspective on regression?\", '2. Can you provide information on regression as discussed in the course?', '3. How does the course cover the topic of regression?', \"4. What are the course's teachings on regression?\", '5. In relation to the course, what is mentioned about regression?']\n"
+     ]
+    },
+    {
+     "data": {
+      "text/plain": [
+       "8"
+      ]
+     },
+     "execution_count": 6,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "# Run\n",
+    "retriever = MultiQueryRetriever(retriever=vectordb.as_retriever(), \n",
+    "                                llm_chain=llm_chain,\n",
+    "                                parser_key=\"lines\") # \"lines\" is the key (attribute name) of the parsed output\n",
+    "\n",
+    "# Results\n",
+    "unique_docs = retriever.get_relevant_documents(question=\"What does the course say about regression?\")\n",
+    "len(unique_docs)"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.9.16"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}

langchain/retrievers/__init__.py

@@ -14,6 +14,7 @@ from langchain.retrievers.llama_index import (
 from langchain.retrievers.merger_retriever import MergerRetriever
 from langchain.retrievers.metal import MetalRetriever
 from langchain.retrievers.milvus import MilvusRetriever
+from langchain.retrievers.multi_query import MultiQueryRetriever
 from langchain.retrievers.pinecone_hybrid_search import PineconeHybridSearchRetriever
 from langchain.retrievers.pupmed import PubMedRetriever
 from langchain.retrievers.remote_retriever import RemoteLangChainRetriever
@@ -43,6 +44,7 @@ __all__ = [
     "MergerRetriever",
     "MetalRetriever",
     "MilvusRetriever",
+    "MultiQueryRetriever",
     "PineconeHybridSearchRetriever",
     "PubMedRetriever",
     "RemoteLangChainRetriever",

langchain/retrievers/multi_query.py

@@ -0,0 +1,158 @@
+import logging
+from typing import List
+
+from pydantic import BaseModel, Field
+
+from langchain.chains.llm import LLMChain
+from langchain.llms.base import BaseLLM
+from langchain.output_parsers.pydantic import PydanticOutputParser
+from langchain.prompts.prompt import PromptTemplate
+from langchain.schema import BaseRetriever, Document
+
+logging.basicConfig(level=logging.INFO)
+
+
+class LineList(BaseModel):
+    lines: List[str] = Field(description="Lines of text")
+
+
+class LineListOutputParser(PydanticOutputParser):
+    def __init__(self) -> None:
+        super().__init__(pydantic_object=LineList)
+
+    def parse(self, text: str) -> LineList:
+        lines = text.strip().split("\n")
+        return LineList(lines=lines)
+
+
+# Default prompt
+DEFAULT_QUERY_PROMPT = PromptTemplate(
+    input_variables=["question"],
+    template="""You are an AI language model assistant. Your task is 
+    to generate 3 different versions of the given user 
+    question to retrieve relevant documents from a vector  database. 
+    By generating multiple perspectives on the user question, 
+    your goal is to help the user overcome some of the limitations 
+    of distance-based similarity search. Provide these alternative 
+    questions seperated by newlines. Original question: {question}""",
+)
+
+
+class MultiQueryRetriever(BaseRetriever):
+
+    """Given a user query, use an LLM to write a set of queries.
+    Retrieve docs for each query. Rake the unique union of all retrieved docs."""
+
+    def __init__(
+        self,
+        retriever: BaseRetriever,
+        llm_chain: LLMChain,
+        verbose: bool = True,
+        parser_key: str = "lines",
+    ) -> None:
+        """Initialize MultiQueryRetriever.
+
+        Args:
+            retriever: retriever to query documents from
+            llm_chain: llm_chain for query generation
+            verbose: show the queries that we generated to the user
+            parser_key: attribute name for the parsed output
+
+        Returns:
+            MultiQueryRetriever
+        """
+        self.retriever = retriever
+        self.llm_chain = llm_chain
+        self.verbose = verbose
+        self.parser_key = parser_key
+
+    @classmethod
+    def from_llm(
+        cls,
+        retriever: BaseRetriever,
+        llm: BaseLLM,
+        prompt: PromptTemplate = DEFAULT_QUERY_PROMPT,
+        parser_key: str = "lines",
+    ) -> "MultiQueryRetriever":
+        """Initialize from llm using default template.
+
+        Args:
+            retriever: retriever to query documents from
+            llm: llm for query generation using DEFAULT_QUERY_PROMPT
+
+        Returns:
+            MultiQueryRetriever
+        """
+        output_parser = LineListOutputParser()
+        llm_chain = LLMChain(llm=llm, prompt=prompt, output_parser=output_parser)
+        return cls(
+            retriever=retriever,
+            llm_chain=llm_chain,
+            parser_key=parser_key,
+        )
+
+    def get_relevant_documents(self, question: str) -> List[Document]:
+        """Get relevated documents given a user query.
+
+        Args:
+            question: user query
+
+        Returns:
+            Unique union of relevant documents from all generated queries
+        """
+        queries = self.generate_queries(question)
+        documents = self.retrieve_documents(queries)
+        unique_documents = self.unique_union(documents)
+        return unique_documents
+
+    async def aget_relevant_documents(self, query: str) -> List[Document]:
+        raise NotImplementedError
+
+    def generate_queries(self, question: str) -> List[str]:
+        """Generate queries based upon user input.
+
+        Args:
+            question: user query
+
+        Returns:
+            List of LLM generated queries that are similar to the user input
+        """
+        response = self.llm_chain({"question": question})
+        lines = getattr(response["text"], self.parser_key, [])
+        if self.verbose:
+            logging.info(f"Generated queries: {lines}")
+        return lines
+
+    def retrieve_documents(self, queries: List[str]) -> List[Document]:
+        """Run all LLM generated queries.
+
+        Args:
+            queries: query list
+
+        Returns:
+            List of retrived Documents
+        """
+        documents = []
+        for query in queries:
+            docs = self.retriever.get_relevant_documents(query)
+            documents.extend(docs)
+        return documents
+
+    def unique_union(self, documents: List[Document]) -> List[Document]:
+        """Get uniqe Documents.
+
+        Args:
+            documents: List of retrived Documents
+
+        Returns:
+            List of unique retrived Documents
+        """
+        # Create a dictionary with page_content as keys to remove duplicates
+        # TODO: Add Document ID property (e.g., UUID)
+        unique_documents_dict = {
+            (doc.page_content, tuple(sorted(doc.metadata.items()))): doc
+            for doc in documents
+        }
+
+        unique_documents = list(unique_documents_dict.values())
+        return unique_documents