Compound index and field order in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Effect of field order in compound index on query performance

Consider a MongoDB collection with a compound index on { age: 1, score: -1 }. Which query will efficiently use this index?

MongoDB

db.collection.find({ age: 25, score: { $gte: 80 } })

A The query uses the index efficiently because it matches the prefix fields in order.

B The query does not use the index because the order of fields in the query is reversed.

C The query uses the index only for the <code>score</code> field, ignoring <code>age</code>.

D The query cannot use the index because it uses a range query on <code>score</code>.

💡 Hint

Think about how MongoDB uses the prefix of the compound index fields in order.

MongoDB uses the compound index starting from the first field. Since the query filters on age first and then on score, it can use the index efficiently.

🧠 Conceptual

intermediate

Understanding field order in compound indexes

Why does the order of fields in a MongoDB compound index matter?

A Because MongoDB ignores the order and uses all fields equally in any query.

B Because MongoDB can only use the index if queries filter on the first field(s) in the order defined.

C Because the order determines the storage size of the index but not query usage.

D Because the order affects only the write speed, not the read speed.

💡 Hint

Think about how indexes are structured and how queries match them.

The order matters because MongoDB uses the index starting from the first field. If a query does not filter on the first field(s), the index cannot be used efficiently.

📝 Syntax

advanced

Correct syntax for creating a compound index with field order

Which of the following commands correctly creates a compound index on lastName ascending and age descending in MongoDB?

A db.users.createIndex({ lastName: 1, age: -1 })

B db.users.createIndex(["lastName", 1], ["age", -1])

C db.users.createIndex({ age: -1, lastName: 1 })

D db.users.createIndex({ lastName: "asc", age: "desc" })

💡 Hint

Remember the syntax for createIndex uses a single object with field names and directions.

The correct syntax uses a single object with field names as keys and 1 or -1 as values to indicate ascending or descending order.

❓ optimization

advanced

Optimizing queries with compound index field order

You have a compound index on { city: 1, status: 1 }. Which query will NOT use this index efficiently?

Options:
A) db.collection.find({ city: "NY", status: "active" })
B) db.collection.find({ status: "active" })
C) db.collection.find({ city: "NY" })
D) db.collection.find({ city: "NY", status: { $in: ["active", "pending"] } })

A Query filters on both city and status, uses index efficiently.

B Query filters only on city, uses index efficiently.

C Query filters only on status, cannot use index efficiently.

D Query filters on city and status with $in, uses index efficiently.

💡 Hint

Recall that the index can only be used efficiently if the query filters on the first field in the index.

Since the index starts with city, queries filtering only on status cannot use the index efficiently.

🔧 Debug

expert

Diagnosing why a compound index is not used

A MongoDB collection has a compound index on { category: 1, price: 1 }. A query db.products.find({ price: { $lt: 100 } }) is slow and does not use the index. Why?

A Because range queries on <code>price</code> prevent index usage.

B Because the query uses a comparison operator which MongoDB does not support in indexes.

C Because the index fields are in ascending order and the query needs descending order.

D Because the query does not filter on the first field <code>category</code>, so the index is not used.

💡 Hint

Think about how MongoDB uses compound indexes and the importance of filtering on the first field.

MongoDB requires queries to filter on the first field of a compound index to use it efficiently. Since the query filters only on price (second field), the index is not used.