Query patterns that cause collection scans in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a collection scan in MongoDB?

A collection scan happens when MongoDB reads every document in a collection to find those that match a query. It is slow because it checks all documents instead of using an index.

beginner

Name a common query pattern that causes a collection scan.

Queries that do not use an index, such as searching on a field without an index or using operators that prevent index use, cause collection scans.

intermediate

Why does using a regular expression without an index cause a collection scan?

Because MongoDB cannot use an index efficiently for regex patterns that do not start with a fixed prefix, it scans all documents to match the pattern.

intermediate

How does querying with $not or $ne operators affect index usage?

Queries using $not or $ne often cause collection scans because these operators do not use indexes well, forcing MongoDB to check all documents.

beginner

What is a way to avoid collection scans in MongoDB queries?

Create indexes on fields used in queries and write queries that can use those indexes, avoiding operators or patterns that prevent index use.

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

A Query using $ne operator on a non-indexed field

B Query using equality on an indexed field

C Query on a field with a supporting index

D Query using _id field

Using $ne on a field without an index causes MongoDB to scan all documents, resulting in a collection scan.

What does MongoDB do during a collection scan?

A Reads documents in random order

B Reads only indexed documents

C Reads all documents in the collection

D Reads only documents matching the query

A collection scan means MongoDB reads every document in the collection to find matches.

Which operator often prevents MongoDB from using an index, causing a collection scan?

A $not

B $eq

C $gt

D $in

The $not operator often prevents index use, leading to collection scans.

How can you reduce collection scans in MongoDB queries?

A Avoid creating indexes

B Use queries that match indexed fields

C Use $ne operator frequently

D Scan all documents manually

Using queries that match indexed fields helps MongoDB use indexes and avoid collection scans.

Why does a regex query without a fixed prefix cause a collection scan?

A Because regex queries never scan collections

B Because regex always uses indexes

C Because regex queries are faster without indexes

D Because MongoDB cannot use indexes efficiently without a fixed prefix

MongoDB cannot use indexes efficiently for regex patterns without a fixed prefix, causing collection scans.

Explain what causes a collection scan in MongoDB and how to avoid it.

Think about how MongoDB finds documents and what happens when no index is used.

Describe how operators like $ne and $not affect MongoDB query performance.

Consider how MongoDB searches documents when these operators are used.