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What are Query Engines?

Query engines process questions over your data and return synthesized answers. Unlike chat engines, query engines:
  • Handle single-turn questions (no chat history)
  • Support complex query patterns (routing, sub-questions)
  • Provide structured responses with source citations
  • Enable advanced retrieval strategies

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Core Query Engine Types

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RetrieverQueryEngine

The fundamental query engine that retrieves and synthesizes: 
import { RetrieverQueryEngine } from "llamaindex";

const retriever = index.asRetriever({ similarityTopK: 5 });

const queryEngine = new RetrieverQueryEngine(retriever);

const response = await queryEngine.query({
  query: "What is the main topic?"
});

console.log(response.toString());

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Simple Query Engine from Index

The easiest way to create a query engine: 
import { VectorStoreIndex, Document } from "llamaindex";

const document = new Document({ text: "Your document text" });
const index = await VectorStoreIndex.fromDocuments([document]);

// Creates a RetrieverQueryEngine internally
const queryEngine = index.asQueryEngine();

const response = await queryEngine.query({
  query: "What does this document discuss?"
});

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SubQuestionQueryEngine

Breaks complex questions into sub-questions: 
import {
  Document,
  QueryEngineTool,
  SubQuestionQueryEngine,
  VectorStoreIndex
} from "llamaindex";

const document = new Document({ text: "Your document text" });
const index = await VectorStoreIndex.fromDocuments([document]);

const queryEngineTools = [
  new QueryEngineTool({
    queryEngine: index.asQueryEngine(),
    metadata: {
      name: "documents",
      description: "Information about the documents"
    }
  })
];

const queryEngine = SubQuestionQueryEngine.fromDefaults({
  queryEngineTools
});

const response = await queryEngine.query({
  query: "How did the topic evolve over time?"
});

console.log(response.toString());

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RouterQueryEngine

Routes queries to the best query engine: 
import {
  RouterQueryEngine,
  VectorStoreIndex,
  SummaryIndex,
  SentenceSplitter,
  Settings
} from "llamaindex";
import { OpenAI } from "@llamaindex/openai";
import { SimpleDirectoryReader } from "@llamaindex/readers/directory";

// Configure settings
Settings.llm = new OpenAI();
Settings.nodeParser = new SentenceSplitter({ chunkSize: 1024 });

// Load documents
const documents = await new SimpleDirectoryReader().loadData({
  directoryPath: "./data"
});

// Create different indices
const vectorIndex = await VectorStoreIndex.fromDocuments(documents);
const summaryIndex = await SummaryIndex.fromDocuments(documents);

// Create router
const queryEngine = RouterQueryEngine.fromDefaults({
  queryEngineTools: [
    {
      queryEngine: vectorIndex.asQueryEngine(),
      description: "Useful for retrieving specific context"
    },
    {
      queryEngine: summaryIndex.asQueryEngine(),
      description: "Useful for summarization questions"
    }
  ]
});

const response = await queryEngine.query({
  query: "Give me a summary of the documents"
});

console.log(response.response);
console.log("Selected:", response.metadata.selectorResult);

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Complete Working Example

Here’s a production-ready query engine: 
import { Document, VectorStoreIndex } from "llamaindex";
import fs from "node:fs/promises";
import { createInterface } from "node:readline/promises";

async function main() {
  const rl = createInterface({ 
    input: process.stdin, 
    output: process.stdout 
  });

  // Check API key
  if (!process.env.OPENAI_API_KEY) {
    console.log("OpenAI API key not found in environment variables.");
    process.env.OPENAI_API_KEY = await rl.question(
      "Please enter your OpenAI API key: "
    );
  }

  // Load document
  const essay = await fs.readFile("./data/essay.txt", "utf-8");
  const document = new Document({ text: essay, id_: "essay" });

  // Create index and query engine
  const index = await VectorStoreIndex.fromDocuments([document]);
  const queryEngine = index.asQueryEngine();

  console.log("\nReady to answer questions!\n");

  // Query loop
  while (true) {
    const query = await rl.question("Query: ");
    const response = await queryEngine.query({ query });
    console.log(response.toString());
    console.log();
  }
}

main().catch(console.error);

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Response Synthesis

Query engines use response synthesizers to combine retrieved chunks:

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Synthesis Modes

import { getResponseSynthesizer } from "llamaindex";

// Compact: Concatenates chunks up to token limit
const compactSynthesizer = getResponseSynthesizer("compact");

// Refine: Iteratively refines answer with each chunk
const refineSynthesizer = getResponseSynthesizer("refine");

// Tree Summarize: Builds answer in tree structure
const treeSynthesizer = getResponseSynthesizer("tree_summarize");

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Using Custom Synthesizers

import { RetrieverQueryEngine, getResponseSynthesizer } from "llamaindex";

const retriever = index.asRetriever();
const synthesizer = getResponseSynthesizer("tree_summarize");

const queryEngine = new RetrieverQueryEngine({
  retriever,
  responseSynthesizer: synthesizer
});

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Streaming Synthesis

Stream responses as they’re generated: 
import { getResponseSynthesizer, NodeWithScore, TextNode } from "llamaindex";

const synthesizer = getResponseSynthesizer("compact");

const nodesWithScore: NodeWithScore[] = [
  {
    node: new TextNode({ text: "Relevant chunk 1" }),
    score: 1.0
  },
  {
    node: new TextNode({ text: "Relevant chunk 2" }),
    score: 0.8
  }
];

const stream = await synthesizer.synthesize(
  {
    query: "What is the answer?",
    nodes: nodesWithScore
  },
  true  // Enable streaming
);

for await (const chunk of stream) {
  process.stdout.write(chunk.response);
}

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Advanced Query Patterns

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Sub-Question Decomposition

Break complex queries into simpler sub-questions: 
import {
  SubQuestionQueryEngine,
  QueryEngineTool,
  VectorStoreIndex,
  Document
} from "llamaindex";

const doc1 = new Document({ 
  text: "Content about topic A",
  id_: "doc_a" 
});
const doc2 = new Document({ 
  text: "Content about topic B",
  id_: "doc_b" 
});

const indexA = await VectorStoreIndex.fromDocuments([doc1]);
const indexB = await VectorStoreIndex.fromDocuments([doc2]);

const queryEngineTools = [
  new QueryEngineTool({
    queryEngine: indexA.asQueryEngine(),
    metadata: {
      name: "topic_a",
      description: "Information about topic A"
    }
  }),
  new QueryEngineTool({
    queryEngine: indexB.asQueryEngine(),
    metadata: {
      name: "topic_b",
      description: "Information about topic B"
    }
  })
];

const queryEngine = SubQuestionQueryEngine.fromDefaults({
  queryEngineTools
});

// Complex query that requires both sources
const response = await queryEngine.query({
  query: "Compare and contrast topic A and topic B"
});

console.log(response.toString());

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Router-Based Selection

Automatically choose the best query engine: 
import { RouterQueryEngine, VectorStoreIndex, SummaryIndex } from "llamaindex";

const vectorIndex = await VectorStoreIndex.fromDocuments(documents);
const summaryIndex = await SummaryIndex.fromDocuments(documents);

const router = RouterQueryEngine.fromDefaults({
  queryEngineTools: [
    {
      queryEngine: vectorIndex.asQueryEngine(),
      description: "Best for specific fact retrieval and targeted questions"
    },
    {
      queryEngine: summaryIndex.asQueryEngine(),
      description: "Best for summarization and overview questions"
    }
  ]
});

// Router automatically selects the appropriate engine
const factResponse = await router.query({
  query: "What was the author's first job?"
});
// Uses vectorIndex

const summaryResponse = await router.query({
  query: "Summarize the author's career"
});
// Uses summaryIndex

console.log("Selected engine:", summaryResponse.metadata.selectorResult);

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Query Engine Configuration

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Retrieval Parameters

const queryEngine = index.asQueryEngine({
  similarityTopK: 10,  // Retrieve top 10 chunks
  // Additional retriever options
});

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Custom Retrievers

import { RetrieverQueryEngine } from "llamaindex";

const retriever = index.asRetriever({
  similarityTopK: 5,
  // Add filters, custom params, etc.
});

const queryEngine = new RetrieverQueryEngine(retriever);

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Post-Processing

Add post-processors to refine results: 
import { 
  RetrieverQueryEngine,
  SimilarityPostprocessor 
} from "llamaindex";

const retriever = index.asRetriever();
const postprocessor = new SimilarityPostprocessor({
  similarityCutoff: 0.7  // Filter out chunks below threshold
});

const queryEngine = new RetrieverQueryEngine({
  retriever,
  nodePostprocessors: [postprocessor]
});

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Choosing the Right Query Engine

EngineUse CaseProsCons
RetrieverQueryEngineGeneral Q&ASimple, fastSingle retrieval strategy
SubQuestionQueryEngineComplex questionsBreaks down problemsMore LLM calls
RouterQueryEngineMultiple data sourcesIntelligent routingRequires good descriptions
Index.asQueryEngine()Quick prototypingEasiest setupLess control

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Low-Level Query Pipeline

For maximum control, build the query pipeline manually: 
import {
  Document,
  SentenceSplitter,
  TextNode,
  NodeWithScore,
  getResponseSynthesizer
} from "llamaindex";

// 1. Parse documents
const nodeParser = new SentenceSplitter({ chunkSize: 512 });
const nodes = nodeParser.getNodesFromDocuments([
  new Document({ text: "Your document text" })
]);

// 2. Simulate retrieval (in practice, use embeddings + vector search)
const nodesWithScore: NodeWithScore[] = [
  {
    node: new TextNode({ text: "Relevant chunk 1" }),
    score: 0.9
  },
  {
    node: new TextNode({ text: "Relevant chunk 2" }),
    score: 0.75
  }
];

// 3. Synthesize response
const synthesizer = getResponseSynthesizer("compact");

const response = await synthesizer.synthesize({
  query: "What is the answer?",
  nodes: nodesWithScore
});

console.log(response.toString());

​
Next Steps

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