Deleting and closing indexes in Elasticsearch - Time & Space Complexity

Time Complexity: Deleting and closing indexes

O(n)

Understanding Time Complexity

When deleting or closing indexes in Elasticsearch, it's important to understand how the time taken grows as the number of indexes increases.

We want to know how the cost changes when more indexes are involved.

Scenario Under Consideration

Analyze the time complexity of the following Elasticsearch commands.

DELETE /my_index

POST /my_index/_close

POST /_all/_close

DELETE /index_1,index_2,index_3
    

This code deletes or closes one or multiple indexes by name or all indexes at once.

Identify Repeating Operations

Look for repeated work done when handling multiple indexes.

• Primary operation: Elasticsearch processes each index separately to delete or close it.
• How many times: Once per index requested in the command.

How Execution Grows With Input

As the number of indexes to delete or close grows, the work grows roughly the same way.

Input Size (n)	Approx. Operations
10	About 10 index operations
100	About 100 index operations
1000	About 1000 index operations

Pattern observation: The time grows linearly as you add more indexes to delete or close.

Final Time Complexity

Time Complexity: O(n)

This means the time to delete or close indexes grows directly in proportion to how many indexes you handle.

Common Mistake

[X] Wrong: "Deleting or closing multiple indexes happens instantly regardless of how many there are."

[OK] Correct: Each index requires its own processing, so more indexes mean more time needed.

Interview Connect

Understanding how operations scale with input size helps you explain system behavior clearly and shows you can think about performance in real situations.

Self-Check

"What if we delete indexes in parallel instead of one by one? How would the time complexity change?"

Practice

1. What happens when you delete an index in Elasticsearch?

easy

A. The index is temporarily hidden but data remains.

B. The index and all its data are permanently removed.

C. The index is locked for editing but still searchable.

D. The index is backed up automatically before removal.

Solution

1. Step 1: Understand the delete operation

   Deleting an index removes it completely from Elasticsearch, including all stored data.

2. Step 2: Compare with closing an index

   Closing an index only frees resources but keeps data safe, unlike deleting.

3. Final Answer:

   The index and all its data are permanently removed. -> Option B

4. Quick Check:

   Delete = permanent removal [OK]

Hint: Delete means gone forever, close means temporary [OK]

Common Mistakes:

• Confusing delete with close operation
• Thinking data is saved after delete
• Assuming delete backs up data automatically

2. Which is the correct syntax to close an index named my_index in Elasticsearch?

easy

A. DELETE /my_index/_close

B. PUT /my_index/_close

C. POST /my_index/_close

D. GET /my_index/_close

Solution

1. Step 1: Identify the HTTP method for closing

   Closing an index uses the POST method with the _close endpoint.

2. Step 2: Verify the endpoint and method

   POST /my_index/_close is the correct syntax to close the index named my_index.

3. Final Answer:

   POST /my_index/_close -> Option C

4. Quick Check:

   Close uses POST with _close [OK]

Hint: Close uses POST method with _close endpoint [OK]

Common Mistakes:

• Using DELETE instead of POST
• Using GET or PUT for closing
• Missing the _close suffix in URL

3. What will be the result of this sequence of commands?

DELETE /test_index
POST /test_index/_close
GET /test_index/_settings

medium

A. Index deleted, error on close, error on get settings

B. Index deleted, close ignored, settings returned

C. Index closed successfully, settings returned

D. Error on DELETE, index remains open, settings returned

Solution

1. Step 1: Analyze DELETE command

   The DELETE command removes test_index permanently.

2. Step 2: Analyze POST close command after deletion

   Since the index is deleted, trying to close it causes an error because it no longer exists.

3. Step 3: Analyze GET settings command

   Getting settings for a deleted index also results in an error as the index is gone.

4. Final Answer:

   Index deleted, error on close, error on get settings -> Option A

5. Quick Check:

   Delete removes index, later commands fail [OK]

Hint: Delete first means index gone, next commands fail [OK]

Common Mistakes:

• Assuming close works after delete
• Expecting settings for deleted index
• Thinking delete only hides index

4. You tried to close an index with POST /myindex/_close but got a 404 error. What is the most likely cause?

medium

A. The index is already closed.

B. You used the wrong HTTP method; DELETE is required.

C. You need to delete the index before closing it.

D. The index myindex does not exist.

Solution

1. Step 1: Understand error causes for closing

   Closing an index fails if the index does not exist in Elasticsearch.

2. Step 2: Check other options

   A 404 specifically indicates the index does not exist. The index is already closed. (already closed) causes a 400 error. POST is the correct method, not DELETE (B). Deleting before closing is not required (D).

3. Final Answer:

   The index myindex does not exist. -> Option D

4. Quick Check:

   Close error usually means index missing [OK]

Hint: Close errors usually mean index missing [OK]

Common Mistakes:

• Using DELETE instead of POST
• Thinking index must be deleted before closing
• Assuming error means index already closed

5. You want to temporarily free resources by closing an index but keep its data safe. Later, you want to reopen it. Which sequence of commands should you use?

hard

A. POST /myindex/_close then POST /myindex/_open

B. DELETE /myindex then POST /myindex/_open

C. POST /myindex/_open then POST /myindex/_close

D. PUT /myindex/_close then DELETE /myindex

Solution

1. Step 1: Close index to free resources

   Use POST /myindex/_close to close the index without deleting data.

2. Step 2: Reopen index when needed

   Use POST /myindex/_open to reopen the closed index and make it searchable again.

3. Final Answer:

   POST /myindex/_close then POST /myindex/_open -> Option A

4. Quick Check:

   Close then open to save and restore resources [OK]

Hint: Close with POST _close, reopen with POST _open [OK]

Common Mistakes:

• Deleting index instead of closing
• Opening before closing
• Using wrong HTTP methods like PUT or DELETE