Pagination pattern with skip and limit in MongoDB - Time & Space Complexity

Time Complexity: Pagination pattern with skip and limit

O(n)

Understanding Time Complexity

When using pagination in MongoDB, we often use skip and limit to get a portion of data.

We want to understand how the time to get results changes as we ask for pages further down the list.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

    db.collection.find().skip(1000).limit(10)
    

This code skips the first 1000 documents and then returns the next 10 documents from the collection.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Skipping over documents before returning the page.
• How many times: The database must scan or traverse each document up to the skip number (e.g., 1000 times).

How Execution Grows With Input

As the skip number grows, the database does more work to reach the starting point of the page.

Input Size (skip n)	Approx. Operations
10	About 10 document scans
100	About 100 document scans
1000	About 1000 document scans

Pattern observation: The work grows roughly in direct proportion to the skip value.

Final Time Complexity

Time Complexity: O(n)

This means the time to get a page grows linearly with how many documents you skip.

Common Mistake

[X] Wrong: "Skipping documents is instant and does not affect performance."

[OK] Correct: The database must still look at each skipped document internally, so skipping many documents takes more time.

Interview Connect

Understanding how pagination works under the hood helps you explain trade-offs and choose better methods in real projects.

Self-Check

"What if we replaced skip with a range query on an indexed field? How would the time complexity change?"

Practice

1.

What does the skip method do in MongoDB pagination?

easy

A. It limits the number of documents returned.

B. It deletes documents from the collection.

C. It sorts the documents in ascending order.

D. It skips a specified number of documents before returning results.

Solution

1. Step 1: Understand the role of skip

   The skip method tells MongoDB to ignore a certain number of documents from the start of the result set.

2. Step 2: Compare with other methods

   limit controls how many documents to return, not skipping. Sorting and deleting are unrelated to skip.

3. Final Answer:

   It skips a specified number of documents before returning results. -> Option D

4. Quick Check:

   skip = skip documents [OK]

Hint: Skip jumps over documents, limit sets how many to show [OK]

Common Mistakes:

• Confusing skip with limit
• Thinking skip sorts documents
• Assuming skip deletes documents

2.

Which of the following is the correct syntax to get the second page of results with 5 documents per page in MongoDB?

db.collection.find().skip(?).limit(?)

easy

A. skip(1).limit(5)

B. skip(10).limit(5)

C. skip(5).limit(5)

D. skip(0).limit(5)

Solution

1. Step 1: Calculate skip value for page 2

   Each page has 5 documents, so page 1 skips 0, page 2 skips 5 documents (5 * (2-1) = 5).

2. Step 2: Set limit to page size

   Limit should be 5 to get 5 documents per page.

3. Final Answer:

   skip(5).limit(5) -> Option C

4. Quick Check:

   Page 2 skip = 5, limit = 5 [OK]

Hint: Skip = pageSize * (pageNumber - 1), limit = pageSize [OK]

Common Mistakes:

• Using skip(10) for page 2
• Setting skip to 1 instead of 5
• Confusing skip and limit values

3.

Given a collection with documents numbered 1 to 10, what documents will this query return?

db.collection.find().skip(3).limit(4)

medium

A. Documents 1 to 4

B. Documents 4 to 7

C. Documents 3 to 6

D. Documents 5 to 8

Solution

1. Step 1: Understand skip(3)

   Skip the first 3 documents: documents 1, 2, and 3 are ignored.

2. Step 2: Apply limit(4)

   Return the next 4 documents after skipping: documents 4, 5, 6, and 7.

3. Final Answer:

   Documents 4 to 7 -> Option B

4. Quick Check:

   Skip 3, then limit 4 = docs 4-7 [OK]

Hint: Skip removes first N, limit picks next M documents [OK]

Common Mistakes:

• Counting skip as inclusive
• Starting limit count before skip
• Off-by-one errors in document numbers

4.

Identify the error in this pagination query to get page 3 with 10 documents per page:

db.collection.find().skip(20).limit(20)

medium

A. limit should be 10, not 20

B. skip value is too high

C. skip should be 30 for page 3

D. No error, query is correct

Solution

1. Step 1: Calculate correct skip and limit for page 3

   Skip should be 10 * (3-1) = 20, which is correct here.

2. Step 2: Check limit value

   Limit should be 10 to get 10 documents per page, but query uses limit(20), which is too many.

3. Final Answer:

   limit should be 10, not 20 -> Option A

4. Quick Check:

   Limit = page size = 10 [OK]

Hint: Limit equals page size, skip = pageSize*(page-1) [OK]

Common Mistakes:

• Setting limit equal to skip
• Using skip as page number
• Assuming no limit needed

5.

You want to display page 4 of a product list with 8 items per page. However, the total products are 30. Which query correctly handles this pagination without returning empty results?

db.products.find().skip(?).limit(?)

hard

A. skip(24).limit(8)

B. skip(32).limit(8)

C. skip(16).limit(8)

D. skip(30).limit(8)

Solution

1. Step 1: Calculate skip for page 4

   Skip = 8 * (4-1) = 24 documents to reach page 4.

2. Step 2: Check total documents and limit

   Total is 30, so skipping 24 leaves 6 documents (30-24). Limit 8 is okay; it will return only available 6 documents without error.

3. Step 3: Analyze other options

   skip(32) and skip(30) exceed total documents, returning empty results. skip(16) is for page 3, not page 4.

4. Final Answer:

   skip(24).limit(8) -> Option A

5. Quick Check:

   Page 4 skip = 24, limit = 8, total 30 [OK]

Hint: Skip = pageSize*(page-1), limit = pageSize, check total docs [OK]

Common Mistakes:

• Skipping beyond total documents
• Using wrong skip for page number
• Ignoring total document count