Retrieving a document by ID in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of retrieving a document by ID in Elasticsearch?

It allows you to quickly find and get a specific document using its unique identifier without searching through the entire index.

beginner

Which HTTP method is used to retrieve a document by ID in Elasticsearch?

The GET method is used to retrieve a document by its ID.

beginner

Show the basic URL format to retrieve a document by ID from an index named 'products' with ID '123'.

GET /products/_doc/123

intermediate

What does Elasticsearch return if the document with the given ID does not exist?

It returns a JSON response with "found": false, indicating the document was not found.

intermediate

Why is retrieving a document by ID faster than searching with a query?

Because Elasticsearch uses an internal index to directly locate the document by its unique ID, avoiding scanning multiple documents.

Which Elasticsearch API endpoint retrieves a document by its ID?

A /index/_update/id

B /index/_search

C /index/_delete/id

D /index/_doc/id

The GET request to /index/_doc/id retrieves the document by its ID.

What HTTP method do you use to get a document by ID?

A GET

B POST

C PUT

D DELETE

GET is used to retrieve data without changing it.

If a document is not found by ID, what key in the response indicates this?

A "found": true

B "found": false

C "exists": true

D "exists": false

The response includes "found": false when the document does not exist.

Which part of the URL specifies the document ID when retrieving by ID?

A The last segment after _doc/

B The document type

C The index name

D The query parameters

The document ID is the last part of the URL after _doc/.

Why is retrieving by ID more efficient than a search query?

A It scans all documents

B It uses a full text search

C It directly accesses the document using its unique ID

D It sorts documents before returning

Retrieving by ID uses a direct lookup, making it faster.

Explain how to retrieve a document by ID in Elasticsearch and what the response looks like.

Think about the URL and the JSON keys in the response.

Describe why retrieving a document by ID is faster than running a search query in Elasticsearch.

Consider how Elasticsearch finds documents internally.

Practice

1. What is the fastest way to retrieve a single document in Elasticsearch?

easy

A. Retrieve the document by its ID using the GET API

B. Search all documents with a match_all query

C. Use a scroll API to iterate over documents

D. Run an aggregation query on the index

Solution

1. Step 1: Understand retrieval methods

   Retrieving by ID directly targets one document, making it the fastest method.

2. Step 2: Compare with other methods

   Search queries and scroll APIs scan multiple documents, which is slower than direct ID retrieval.

3. Final Answer:

   Retrieve the document by its ID using the GET API -> Option A

4. Quick Check:

   Fastest retrieval = GET by ID [OK]

Hint: Use GET API with document ID for fastest retrieval [OK]

Common Mistakes:

• Thinking search queries are faster than direct ID retrieval
• Using scroll API for single document retrieval
• Confusing aggregation with document retrieval

2. Which of the following is the correct syntax to retrieve a document by ID in Elasticsearch?

easy

A. GET /index/_doc/123

B. GET /index/_search {"query": {"term": {"_id": "123"}}}

C. POST /index/_doc/123/_get

D. GET /index/_search/123

Solution

1. Step 1: Recall the GET by ID syntax

   The correct syntax uses GET with the index and document ID: GET /index/_doc/123

2. Step 2: Identify incorrect options

   GET /index/_search {"query": {"term": {"_id": "123"}}} uses a search query, not direct GET by ID. POST /index/_doc/123/_get uses POST and an invalid path. GET /index/_search/123 has an invalid URL structure.

3. Final Answer:

   GET /index/_doc/123 -> Option A

4. Quick Check:

   Correct syntax = GET /index/_doc/ID [OK]

Hint: Use GET /index/_doc/{id} to retrieve document by ID [OK]

Common Mistakes:

• Using search API instead of GET by ID
• Wrong HTTP method like POST
• Incorrect URL path structure

3. Given the following request, what will Elasticsearch return if the document with ID 'abc123' exists?

GET /products/_doc/abc123

medium

A. HTTP 404 Not Found

B. {"_index":"products","_id":"abc123","found":false}

C. {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}}

D. {"error":"Document not found"}

Solution

1. Step 1: Understand the GET by ID response

   If the document exists, Elasticsearch returns found: true with the document source.

2. Step 2: Match the response format

   {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}} shows found: true and the document data, which matches expected output.

3. Final Answer:

   {"_index":"products","_id":"abc123","found":true,"_source":{"name":"Laptop","price":1200}} -> Option C

4. Quick Check:

   Document exists = found: true with _source [OK]

Hint: Found true means document exists with _source data [OK]

Common Mistakes:

• Expecting HTTP 404 instead of found: false
• Confusing error message with standard response
• Ignoring the _source field in response

4. You run this request to get a document by ID but get an error:

GET /store/_doc

What is the problem and how to fix it?

medium

A. The _doc endpoint does not support GET requests

B. Wrong HTTP method; use POST instead of GET

C. Index name is incorrect; change 'store' to '_store'

D. Missing document ID in URL; add the ID after _doc

Solution

1. Step 1: Identify the request format

   The GET by ID request requires the document ID after _doc in the URL.

2. Step 2: Fix the URL

   Add the document ID after _doc, e.g., GET /store/_doc/123

3. Final Answer:

   Missing document ID in URL; add the ID after _doc -> Option D

4. Quick Check:

   GET by ID needs document ID in URL [OK]

Hint: Always include document ID after _doc in GET URL [OK]

Common Mistakes:

• Omitting document ID in GET request URL
• Using wrong HTTP method
• Misnaming the index with underscore

5. You want to retrieve a document by ID but also handle the case when the document is missing without causing an error. Which approach is best?

hard

A. Use a search query with term _id and expect zero hits if missing

B. Use GET /index/_doc/id and check if 'found' is true in the response

C. Use GET /index/_doc/id and catch HTTP 404 error to handle missing

D. Use scroll API to find the document and handle empty results

Solution

1. Step 1: Understand GET by ID response behavior

   GET by ID returns a JSON with 'found': false if the document is missing, no error thrown.

2. Step 2: Compare with other methods

   Search queries return hits but are slower; catching HTTP 404 is unnecessary; scroll API is for multiple docs.

3. Final Answer:

   Use GET /index/_doc/id and check if 'found' is true in the response -> Option B

4. Quick Check:

   Check 'found' field to detect missing document [OK]

Hint: Check 'found' in GET response to detect missing docs safely [OK]

Common Mistakes:

• Expecting HTTP 404 error for missing documents
• Using search queries instead of GET by ID
• Using scroll API for single document retrieval