Indexing a document (POST/PUT) in Elasticsearch - Time & Space Complexity

Time Complexity: Indexing a document (POST/PUT)

O(n)

Understanding Time Complexity

When we add or update a document in Elasticsearch, it takes some time to process. Understanding how this time grows helps us plan for bigger data.

We want to know: How does the time to index a document change as the document size or system load grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

POST /my_index/_doc/1
{
  "user": "alice",
  "message": "Hello Elasticsearch!",
  "timestamp": "2024-06-01T12:00:00Z"
}
    

This code adds a new document with some fields into the index called my_index.

Identify Repeating Operations

Look at what happens when indexing:

• Primary operation: Parsing and analyzing each field in the document.
• How many times: Once per field, so the number of fields matters.

How Execution Grows With Input

As the document gets bigger with more fields or larger text, the work grows roughly in direct proportion.

Input Size (fields)	Approx. Operations
10	10 units of parsing and indexing
100	100 units of parsing and indexing
1000	1000 units of parsing and indexing

Pattern observation: Doubling the number of fields roughly doubles the work needed.

Final Time Complexity

Time Complexity: O(n)

This means the time to index grows linearly with the number of fields or size of the document.

Common Mistake

[X] Wrong: "Indexing a document always takes the same time no matter its size."

[OK] Correct: Larger documents have more fields and text to process, so they take more time to index.

Interview Connect

Knowing how indexing time grows helps you design efficient data flows and handle bigger data smoothly. This skill shows you understand real system behavior.

Self-Check

"What if we indexed documents with nested objects or arrays? How would the time complexity change?"

Practice

1. What is the main difference between using POST and PUT when indexing a document in Elasticsearch?

easy

A. POST adds a document with an auto-generated ID, PUT adds or replaces a document with a specified ID.

B. POST deletes a document, PUT updates a document.

C. POST is used only for searching, PUT is used for indexing.

D. POST requires an ID, PUT never requires an ID.

Solution

1. Step 1: Understand POST behavior

   POST indexes a document and Elasticsearch generates a unique ID automatically.

2. Step 2: Understand PUT behavior

   PUT indexes a document with the ID you specify, replacing any existing document with that ID.

3. Final Answer:

   POST adds a document with an auto-generated ID, PUT adds or replaces a document with a specified ID. -> Option A

4. Quick Check:

   Method difference = auto ID vs specified ID [OK]

Hint: Remember: POST auto-IDs, PUT needs your ID [OK]

Common Mistakes:

• Thinking POST deletes documents
• Confusing PUT with search operation
• Assuming PUT never requires an ID

2. Which of the following is the correct syntax to index a document with ID "123" using PUT in Elasticsearch?

easy

A. POST /index/_doc {"field": "value"}

B. POST /index/_doc/123 {"field": "value"}

C. PUT /index/_doc {"field": "value"}

D. PUT /index/_doc/123 {"field": "value"}

Solution

1. Step 1: Identify correct HTTP method and URL

   To specify an ID, use PUT with the URL including the ID: /index/_doc/123.

2. Step 2: Confirm request body format

   The body contains the document fields as JSON, e.g., {"field": "value"}.

3. Final Answer:

   PUT /index/_doc/123 {"field": "value"} -> Option D

4. Quick Check:

   PUT + ID in URL = correct syntax [OK]

Hint: PUT with ID in URL, POST without ID [OK]

Common Mistakes:

• Using POST with an ID in URL
• Omitting the ID when using PUT
• Confusing URL structure

3. Given this request to index a document:

POST /products/_doc
{
  "name": "Coffee Mug",
  "price": 12.99
}

What will Elasticsearch do after this request?

medium

A. Index the document with an auto-generated ID under the 'products' index.

B. Replace any existing document with ID 'products'.

C. Return an error because ID is missing.

D. Create a new index named '_doc'.

Solution

1. Step 1: Analyze the HTTP method and URL

   The request uses POST on /products/_doc, which means index 'products' and type '_doc'.

2. Step 2: Understand ID assignment

   Since no ID is specified, Elasticsearch generates a unique ID automatically and indexes the document.

3. Final Answer:

   Index the document with an auto-generated ID under the 'products' index. -> Option A

4. Quick Check:

   POST without ID = auto-generated ID [OK]

Hint: POST without ID means auto ID assigned [OK]

Common Mistakes:

• Thinking missing ID causes error
• Assuming document replaces existing one
• Confusing index and type names

4. You tried to index a document with PUT /myindex/_doc without specifying an ID. What is the problem with this request?

medium

A. It will create a document with a random ID.

B. It will update all documents in the index.

C. It will cause a syntax error because ID is missing in the URL.

D. It will delete the entire index.

Solution

1. Step 1: Check the HTTP method and URL requirements

   PUT requires an explicit document ID in the URL to index or replace a document.

2. Step 2: Identify the error cause

   Since the ID is missing, Elasticsearch will reject the request with an error due to invalid URL syntax.

3. Final Answer:

   It will cause a syntax error because ID is missing in the URL. -> Option C

4. Quick Check:

   PUT needs ID in URL [OK]

Hint: PUT without ID in URL causes error [OK]

Common Mistakes:

• Assuming PUT auto-generates ID
• Thinking it deletes index
• Believing it updates all documents

5. You want to update a document if it exists or create it if it doesn't, using ID "789" in index "users". Which is the best way to do this in Elasticsearch?

hard

A. Use POST /users/_doc with the document body {"id": "789"}.

B. Use PUT /users/_doc/789 with the document body.

C. Use POST /users/_doc without ID.

D. Use DELETE /users/_doc/789 then POST /users/_doc.

Solution

1. Step 1: Understand update or create behavior

   PUT with a specified ID will create the document if missing or replace it if it exists.

2. Step 2: Compare options

   POST without ID creates new documents with auto IDs, so it won't update by ID. Deleting then posting is inefficient.

3. Final Answer:

   Use PUT /users/_doc/789 with the document body. -> Option B

4. Quick Check:

   PUT with ID updates or creates [OK]

Hint: PUT with ID updates or creates document [OK]

Common Mistakes:

• Specifying ID in the document body
• Omitting ID when update needed
• Deleting before posting wastes resources