Deleting documents in Elasticsearch - Time & Space Complexity

Time Complexity: Deleting documents

O(n)

Understanding Time Complexity

When deleting documents in Elasticsearch, it's important to know how the time taken changes as more documents are involved.

We want to understand how the cost grows when deleting one or many documents.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

POST /my-index/_delete_by_query
{
  "query": {
    "term": { "status": "inactive" }
  }
}
    

This code deletes all documents in "my-index" where the status is "inactive".

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Elasticsearch scans documents matching the query and deletes each one.
• How many times: Once for each matching document in the index.

How Execution Grows With Input

As the number of documents matching the query grows, the number of delete operations grows too.

Input Size (n)	Approx. Operations
10	About 10 delete checks and deletes
100	About 100 delete checks and deletes
1000	About 1000 delete checks and deletes

Pattern observation: The work grows roughly in direct proportion to the number of documents to delete.

Final Time Complexity

Time Complexity: O(n)

This means the time to delete documents grows linearly with how many documents match the query.

Common Mistake

[X] Wrong: "Deleting documents is always instant regardless of how many match."

[OK] Correct: Each matching document must be found and deleted, so more matches mean more work and more time.

Interview Connect

Understanding how deletion scales helps you explain performance in real systems and shows you can think about costs beyond just writing code.

Self-Check

"What if we changed the query to delete documents by ID one at a time? How would the time complexity change?"

Practice

1.

Which Elasticsearch API method is used to delete a single document by its ID?

easy

A. GET method with document ID

B. DELETE method with document ID

C. POST method with document ID

D. PUT method with document ID

Solution

1. Step 1: Understand document deletion in Elasticsearch

   Deleting a single document requires specifying its ID and using the DELETE HTTP method.

2. Step 2: Identify the correct API method

   The DELETE method with the document ID is the standard way to remove one document.

3. Final Answer:

   DELETE method with document ID -> Option B

4. Quick Check:

   Delete single document = DELETE with ID [OK]

Hint: Use DELETE with document ID to remove one document fast [OK]

Common Mistakes:

• Using GET or POST instead of DELETE
• Trying to delete without specifying document ID
• Confusing _delete_by_query with single document deletion

2.

Which of the following is the correct syntax to delete a document with ID 123 from index products using Elasticsearch REST API?

?

easy

A. GET /products/_delete/123

B. POST /products/_doc/123/_delete

C. DELETE /products/_doc/123

D. PUT /products/_doc/123

Solution

1. Step 1: Recall the REST API syntax for deleting a document

   The correct syntax uses DELETE method on the path /index/_doc/document_id.

2. Step 2: Match the syntax with the options

   DELETE /products/_doc/123 matches DELETE /products/_doc/123 which is correct.

3. Final Answer:

   DELETE /products/_doc/123 -> Option C

4. Quick Check:

   Delete syntax = DELETE /index/_doc/id [OK]

Hint: DELETE + /index/_doc/document_id is the right syntax [OK]

Common Mistakes:

• Using POST or GET instead of DELETE
• Wrong URL path like _delete or missing _doc
• Using PUT which is for creating/updating

3.

Given this Elasticsearch _delete_by_query request to delete documents where status is "inactive":

{
  "query": {
    "term": { "status": "inactive" }
  }
}

What will be the result if there are 3 documents matching this query?

medium

A. Deletes 3 documents and returns success count 3

B. Deletes 0 documents because term query is incorrect

C. Deletes all documents in the index

D. Returns an error due to missing document ID

Solution

1. Step 1: Understand _delete_by_query behavior

   It deletes all documents matching the query, here documents with status "inactive".

2. Step 2: Apply to given query and data

   Since 3 documents match, all 3 will be deleted and success count will be 3.

3. Final Answer:

   Deletes 3 documents and returns success count 3 -> Option A

4. Quick Check:

   _delete_by_query deletes matching docs = 3 [OK]

Hint: Delete by query removes all matching docs, not just one [OK]

Common Mistakes:

• Thinking _delete_by_query needs document ID
• Assuming it deletes all documents regardless of query
• Confusing term query syntax

4.

What is wrong with this Elasticsearch delete request?

DELETE /users/_doc

It aims to delete a document but fails. Why?

medium

A. Missing document ID in the URL

B. Using DELETE instead of POST

C. Index name is incorrect

D. Missing request body with query

Solution

1. Step 1: Check the delete request URL format

   Deleting a document requires the document ID after _doc in the URL.

2. Step 2: Identify the missing part

   The request lacks the document ID, so Elasticsearch cannot identify which document to delete.

3. Final Answer:

   Missing document ID in the URL -> Option A

4. Quick Check:

   Delete URL needs document ID [OK]

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

Common Mistakes:

• Omitting document ID in delete URL
• Confusing DELETE with POST for deletion
• Thinking request body is needed for single delete

5.

You want to delete all documents from the orders index where the amount field is less than 100. Which Elasticsearch request correctly achieves this?

hard

A. DELETE /orders/_delete_by_query { "query": { "term": { "amount": 100 } } }

B. DELETE /orders/_doc { "query": { "range": { "amount": { "lt": 100 } } } }

C. POST /orders/_doc/amount_lt_100/_delete

D. POST /orders/_delete_by_query { "query": { "range": { "amount": { "lt": 100 } } } }

Solution

1. Step 1: Identify the correct API for deleting multiple documents by condition

   _delete_by_query is used with POST to delete documents matching a query.

2. Step 2: Check the query for amount less than 100

   The range query with "lt": 100 correctly matches documents with amount less than 100.

3. Step 3: Verify the request method and path

   POST /orders/_delete_by_query with the correct JSON body is the proper syntax.

4. Final Answer:

   POST /orders/_delete_by_query { "query": { "range": { "amount": { "lt": 100 } } } } -> Option D

5. Quick Check:

   Delete by query with range and POST = correct [OK]

Hint: Use POST _delete_by_query with range query for conditional deletes [OK]

Common Mistakes:

• Using DELETE method with request body (not supported)
• Using _doc endpoint instead of _delete_by_query
• Using term query instead of range for less than condition