Query patterns that cause collection scans in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the field 'age' is exactly 30.

MongoDB

db.users.find({"age": [1])

Drag options to blanks, or click blank then click option'

A{ $eq: 30 }

B"30"

C{ age: 30 }

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string '30' instead of number 30 causes no match if the field is numeric.

Using a nested object incorrectly inside the value.

The query looks for documents where the 'age' field equals the number 30. Using 30 directly matches the numeric value.

2fill in blank

medium

Complete the code to find documents where the 'name' field starts with 'A'.

MongoDB

db.users.find({"name": [1])

Drag options to blanks, or click blank then click option'

A/^A/

B"A%"

C{ $regex: 'A' }

D{ $startsWith: 'A' }

Attempts:

3 left

💡 Hint

Common Mistakes

Using SQL-like wildcards such as 'A%' which MongoDB does not understand.

Using incorrect operators like $startsWith which do not exist.

The regular expression /^A/ matches strings starting with 'A'. This is the correct way to query for names starting with 'A' in MongoDB.

3fill in blank

hard

Fix the error in the query to find documents where 'score' is greater than 50.

MongoDB

db.scores.find({"score": { [1] 50 } })

Drag options to blanks, or click blank then click option'

A$lt:

B$gte:

C$gt:

D$eq:

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gte includes equal values, which is not the same as strictly greater than.

Using $lt or $eq will not find values greater than 50.

The operator $gt means 'greater than'. It must be used as a key in an object to compare values.

4fill in blank

hard

Fill both blanks to create a query that finds documents where 'status' is 'active' and 'age' is less than 40.

MongoDB

db.users.find({"status": [1], "age": { [2] 40 } })

Drag options to blanks, or click blank then click option'

A"active"

B$lt:

C$gt:

D"inactive"

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $lt for age.

Using 'inactive' instead of 'active' for status.

The 'status' field should match the string 'active'. The 'age' field uses the $lt operator to find values less than 40.

5fill in blank

hard

Fill all three blanks to create a query that finds documents where the 'category' is 'books', 'price' is greater than 20, and 'inStock' is true.

MongoDB

db.products.find({"category": [1], "price": { [2] 20 }, "inStock": [3])

Drag options to blanks, or click blank then click option'

A"books"

B$gt:

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using false instead of true for inStock.

Using $lt instead of $gt for price.

The 'category' field matches the string 'books'. The 'price' field uses $gt to find prices greater than 20. The 'inStock' field is a boolean true.

Practice

1. Which of the following query patterns in MongoDB is most likely to cause a collection scan?

easy

A. Querying a field without an index

B. Querying a field with a single-field index

C. Using an indexed field with equality operator

D. Using a covered index query

Solution

1. Step 1: Understand collection scan causes

   A collection scan happens when MongoDB must check every document because no index supports the query.

2. Step 2: Analyze query patterns

   Querying a field without an index forces MongoDB to scan the entire collection, unlike queries using indexed fields.

3. Final Answer:

   Querying a field without an index -> Option A

4. Quick Check:

   No index = collection scan [OK]

Hint: No index on field? Expect collection scan [OK]

Common Mistakes:

• Assuming all queries use indexes automatically
• Confusing covered index with collection scan
• Thinking equality queries always cause scans

2. Which of the following MongoDB query syntaxes is correct and avoids collection scans by using an index on the field age?

easy

A. db.users.find({ age: { $gt: 25 } })

B. db.users.find({ $where: 'this.age > 25' })

C. db.users.find({ age: { $regex: '^2' } })

D. db.users.find({ $text: { $search: '25' } })

Solution

1. Step 1: Check query syntax correctness

   All queries are syntactically valid, but some use operators that prevent index use.

2. Step 2: Identify index-friendly query

   Query with { age: { $gt: 25 } } uses range operator on indexed field, avoiding collection scan. Queries with $where or $regex often cause scans.

3. Final Answer:

   db.users.find({ age: { $gt: 25 } }) -> Option A

4. Quick Check:

   Range query on indexed field = no scan [OK]

Hint: Use simple operators on indexed fields to avoid scans [OK]

Common Mistakes:

• Using $where causes collection scan
• Assuming $regex always uses index
• Confusing $text search with field index

3. Given a collection products with an index on category, what will be the result of this query?

db.products.find({ category: { $in: ['electronics', 'books'] } })

medium

A. Throws a syntax error due to $in usage

B. Performs a collection scan and returns all documents

C. Returns no documents because $in is unsupported

D. Uses index on category and returns matching documents

Solution

1. Step 1: Understand $in operator with indexes

   The $in operator can use indexes on the queried field to find matching values efficiently.

2. Step 2: Analyze query behavior

   Since category is indexed, MongoDB uses the index to find documents where category is 'electronics' or 'books'.

3. Final Answer:

   Uses index on category and returns matching documents -> Option D

4. Quick Check:

   $in + indexed field = index used [OK]

Hint: Indexed field + $in uses index, no scan [OK]

Common Mistakes:

• Thinking $in disables index use
• Assuming $in causes syntax error
• Believing collection scan always happens

4. You have a MongoDB collection orders with an index on status. Which query will cause a collection scan and how can you fix it?

db.orders.find({ $where: 'this.status === "shipped"' })

medium

A. Syntax error; fix by removing $where

B. Uses index; no fix needed

C. Causes collection scan; fix by using { status: 'shipped' }

D. Causes collection scan; fix by adding $regex

Solution

1. Step 1: Identify $where impact on query

   The $where operator runs JavaScript on each document, forcing a collection scan regardless of indexes.

2. Step 2: Fix query to use index

   Replacing $where with a direct field query { status: 'shipped' } uses the index on status, avoiding collection scan.

3. Final Answer:

   Causes collection scan; fix by using { status: 'shipped' } -> Option C

4. Quick Check:

   $where disables index; direct query fixes [OK]

Hint: Avoid $where; use direct field queries for indexes [OK]

Common Mistakes:

• Assuming $where uses indexes
• Trying to fix with $regex instead of direct query
• Confusing syntax error with collection scan

5. You want to optimize this MongoDB query to avoid collection scans on a large users collection:

db.users.find({ $or: [ { age: { $gt: 30 } }, { name: /John/ } ] })

Given indexes on age and name, which change will prevent collection scans?

hard

A. Remove indexes and rely on collection scan for speed

B. Rewrite query to separate $or into two queries using indexed fields

C. Use $where with JavaScript to filter documents

D. Change regex to case-insensitive without anchors

Solution

1. Step 1: Understand $or with mixed operators

   Regex queries often prevent index use, causing collection scans even if other fields are indexed.

2. Step 2: Optimize by splitting queries

   Running two separate queries--one on age and one on name--each using indexes, then combining results in application avoids scans.

3. Final Answer:

   Rewrite query to separate $or into two queries using indexed fields -> Option B

4. Quick Check:

   Split queries + indexes avoid scans [OK]

Hint: Split $or queries to use indexes separately [OK]

Common Mistakes:

• Believing regex always uses index
• Using $where increases scans
• Removing indexes reduces performance